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Extending ITA 2

The previous page dealt with UTF-8, a scheme for extending the ASCII character set to embrace all of UNICODE, and a suggestion of mine to make use of some codes it does not use in order to make it more efficient with regard to bandwith, at the expense of some of its desirable and elegant properties.

On this page, I first describe the current official standard for transmitting lower-case characters within the framework of 5-level teletypewriter code, and then I proceed to propose a compatible scheme of providing for representing, with reasonable efficiency, the entire UNICODE repertoire, divided between character sets for different languages, within 5-level code.

CCITT/ITU Recommendation S.2

The ASCII over AMTOR code shown in the chart of 5-level code representations and variants on the introductory page of the chapter on telecipher machines illustrated one method of transmitting lower-case characters by means of 5-level code, using the all-zeroes character as an extra shift character to access them. There is also an official standard of very recent vintage for using lowercase with 5-level code, which works on a significantly different principle: a LTRS code while already in letters case is used to toggle between upper and lower case.

I'm not kidding, and, no, I didn't make this up myself; this standard is CCITT Recommendation S.2 (Melbourne, 1988), also adopted as ITU-T Recommendation S.2. (It was very hard to locate it again on the Web, after having originally found it by accident.)

This standard does not include ASCII graphics characters, but it was designed to be compatible with the use of the all-zeroes code for supplementary alphabetic characters; these characters could have their lower case available using their shift character in the same fashion as LTRS is used.

This new standard works as follows:

Initially, in the absence of any unusual combinations of FIGS and LTRS codes, the FIGS code switches to printing special characters, and the LTRS code switches to printing capital letters. This is called upper-case mode.

When one is already in letters case, a superfluous LTRS code always switches from upper-case printing to lower-case printing and back again. When in upper-case mode, it also sets lower-case mode. In lower-case mode, whether one is printing upper-case or lower-case letters at the time one has switched to printing figures characters, when one returns from printing figures characters to print letters, one begins with lower-case letters. A superfluous LTRS code encountered when printing lower-case letters switches one to printing upper-case letters, but one remains in lower-case mode, so that a LTRS code exiting from printing figures-case characters still returns one, initially, to printing lower-case letters.

It is only the sequence FIGS LTRS that operates as a reset into upper-case mode. In normal upper-case mode, when returning to letters case from figures case, one is returning to upper-case letters.

This can be a bit confusing, so I will illustrate it with an example:

ABC [FIGS] 1234
[LTRS] DEF [LTRS] ghi [FIGS] 1234 [LTRS] jkl
[LTRS] MNOPQ [FIGS] 1234 [LTRS] rst
[FIGS][LTRS] UVW [FIGS] 1234 [LTRS] XYZ

Essentially, toggling between upper and lower case with a superfluous LTRS is always on. FIGS LTRS resets (to upper-case, or capitals) only the default letters case that a normal LTRS, used for exiting figures printing, returns to. And that default flips back to lower case the first time lower case is accessed with an (otherwise) superflous LTRS.

Thus, this example proceeds as follows:

ABC [FIGS] 1234 [LTRS] DEF

One begins by having only figures and upper-case letters available.

[LTRS] ghi [FIGS] 1234 [LTRS] jkl

The superfluous LTRS now switches one into lower-case mode, as well as immediately switching to printing lower-case letters. The FIGS shift still takes you to normal figures case, and a LTRS shift returns you to lower-case letters.

[LTRS] MNOPQ [FIGS] 1234 [LTRS] rst

A superfluous LTRS shift changes you to printing upper-case characters, but the mode remains lower-case mode. Thus, FIGS takes you to printing digits, and LTRS takes you to printing in the default case for the current mode, which is lower case.

[FIGS][LTRS] UVW [FIGS] 1234 [LTRS] XYZ

A superfluous LTRS toggles between printing upper-case and lower-case, but only moves you from upper-case mode to lower-case mode. To change mode in the reverse direction, the combination FIGS LTRS is required. Once that combination is used, not only do you print in upper-case, but a LTRS shift used after printing figures will return you to the new default case, which is again upper case.

Note that it would be possible to use a superflous third-shift character to toggle between upper- and lower- case Cyrillic; the upper-case/lower-case status of Latin and Cyrillic characters could be independent, or could be joined. Additionally, since with the Cyrillic character set, and with even a number of national-use variants for languages using the Latin character set, some additional letters are placed in the figures-shift repertoire, one could even use a superfluous figures-shift character for toggling the upper/lower case status while in figures mode.

It is also noted in that standard that some teletypewriters might normally ignore the superfluous LTRS code for a higher degree of compatibility with existing equipment, and be awakened into printing in lower case, as well as into lower-case mode, only with the sequence FIGS LTRS LTRS which consists of the FIGS LTRS reset into upper-case mode followed immediately by a superfluous LTRS code.

Towards a Larger Repertoire

The bulk of a typical text document is lower-case letters and spaces. Thus, it would seem that if one is using a fixed number of bits per character, a five-bit code with occasional shifts is optimal from the viewpoint of bandwidth efficiency.

The problem with using a five-bit code is to have available enough reasonably short shift codes so as to provide a large enough character repertoire for any use. If one is constrained to maintain compatibility with ITA 2, one does have the additional limitation that two character positions are used for the carriage return and line feed; in an ideal character code, they might be combined into a newline character, or relegated to some shifted character repertoire.

The use of a superfluous LTRS character to switch between upper and lower case made it possible, within ITA recommendation S.2, to add lower case to ITA 2 without having to use the only 5-bit combination in ITA 2 that remained unused, 00000, or Character no. 32. Of course, it would have been preferable to leave it as a null character, as it corresponds to unpunched paper tape. The combination 11111, which would have been attractive as another ignored character, is used as the next best thing, the letters shift.

Since we are going to use 00000 to access additional characters, it will be referred to below as ALT.

Several possible ways to modify ITA 2 for either an increased character repertoire or increased bandwith efficiency come to mind:

Type I

If the character set of ITA 2, with three national-use characters, and lower-case, is adequate, then the ALT character can be used to save bandwidth as follows:

When printing lower case letters, an ALT preceding a letter could make it uppercase without shifting, and thus requiring a shift back out.

When printing upper case letters, an ALT could be used to select a single figures-shift character.

Since ALT is used as an escape for printable characters, ALT FIGS, ALT LTRS, and ALT ALT, for example, would be available for special purposes, such as switching to other character coding schemes.

Type II

While ALT is the only unused 5-bit code, there are three national-use positions in the figures case, the shifted counterparts of the letters F, G, and H. These could be used to shift into three additional ensembles of characters; for example, a set of extra letters, such as accented letters, a set of extra punctuation marks and other special symbols, and a set of control characters.

This wouldn't involve the use of the ALT code at all, and by using the national-use characters in this way, the resulting code need not favor any existing assignment of those characters.

Type III

Many countries that use a script other than the Latin alphabet use the ALT character as a third shift into their alphabet, so that their teleprinters can still use the Latin alphabet.

In this case, when within this other alphabet, a superfluous ALT character could be used in the same way as a superfluous LTRS character to switch to its lowercase set. When shifting to an alphabet from either the figures case or the other alphabet, the case should begin at the case defined by the current mode, and there should only be a single mode setting for both alphabets. FIGS ALT as well as FIGS LTRS should cause a reset to upper case mode, as well as a shift to the alphabet indicated by the last shift character.

Type IV

The ASCII over AMTOR scheme uses the ALT character in a way that increases compatibility with terminals that ignore it; it does not change whether letters or special characters are printed, it merely shifts to a second set of letters or special characters, respectively, when in letters or figures case.

This allows four, rather than three, sets of 26 printable characters to be used. However, that specific scheme uses lower case as its additional set of letters, and Recommendation S.2 already provides a different method of accessing lower case.

If the second set of letters characters is a national alphabet, then when shifting into figures case, it would make sense to still use the normal figures characters, not the supplementary set, but to return to the national alphabet when shifting into letters case.

Thus, the ALT character, when used while in letters case, should toggle a persistent value indicating which alphabet is in use, but which group of figures characters is used should always be reset to the default group when outside of figures case.

The combination ALT LTRS is clearly available for use; and if the alphabet in use is toggled persistently, a code for resetting to the default alphabet is needed.

Type V

It may be noted that, in some existing arrangements, extra letters for a non-Latin alphabet with more than 26 letters, or accented letters for a language using the Latin alphabet, occupy national-use positions in the figures case. As we have seen, the use of ALT as an escape character, rather than as a shift, furthers bandwidth efficiency, since unlike a shift, no second character for shifting back is needed, when an isolated character from a supplementary group is required. In addition, it would seem that shifting between the Latin alphabet and a non-Latin alphabet is an infrequent operation, and that additional special characters are likely to be largely used in isolation.

Thus, it appeared to me that it would be sensible to use a two-shift code to switch between national alphabets, and use ALT as an escape for supplementary characters, perhaps along the following lines:

When in figures case, an ALT preceding a printable character causes a character from a supplementary set of special characters to be used.

When in letters case, an ALT preceding a letter causes a character from a supplementary set of letters to be used.

The combination ALT LTRS would be used to toggle between national alphabets. With regard to being in upper or lower case, it should not have any effect at all. When it is encountered in figures case, though, it would shift into the next national alphabet, and, like a letters shift, it would select the case that corresponds to the default case for the current mode. (Note that when the national alphabet is different, the characters in the supplementary set of letters are also different.)

The combination ALT FIGS would be used to toggle between different supplementary sets of special characters; it would not shift to printing characters from that set, it would only change what characters were caused to print in future by an ALT character used as an escape.

The combination ALT FIGS LTRS should still be available, and would allow resetting to the default national alphabet. (Ending in LTRS, it also must function as a letters shift.)

The combination ALT LTRS FIGS is also available, and would be used to reset to the default supplementary set of special characters. (Ending in FIGS, it must also function as a figures shift.)

When already in figures shift, in addition to the ALT character, it is possible to assign a function to a superfluous FIGS shift. As it might be useful to print an extended sequence of supplementary figures shift characters, depending on the use to which they were being put, I had started from that point, only allowing switching between supplementary figures shift character sets when shifted into printing from the current supplementary figures shift character set. The ALT character was used for that function when in that mode. While using single-character codes as much as possible is efficient, as a starting point it made the scheme more difficult to understand than it needed to be.

A Simple Alternative

In the introduction to the section on teletypewriter ciphers, the six-bit code used for teletypesetters, which had 5-level code as its basis, was shown. The sixth bit allowed letters and figures to have separate codes, so that a letters shift and a figures shift were not needed, but there were shift and unshift codes to increase the repertoire of figures characters as well as to allow lowercase letters.

This illustrates how one might define a 5-level code inspired by that 6-level code:

Instead of strictly extending ITA 2, some incompatibilities are allowed. To add a "shift" code and an "unshift" code to the existing letters shift and figures shift, now still required, it is necessary to free up an additional code point. This is done by using the code for carriage return to instead represent newline, so that an additional code for line feed is not needed.

Because the shift and unshift codes double the number of figures characters, some positions can be used for other purposes and still leave space for the printable characters used by the teletypesetter code. Thus, codes for line feed and carriage return are given, in case those functions do occasionally have to be separated; a code is given for bell, and one for em space and one for en space.

As the teletypesetter code included a large number of additional control functions, perhaps the code indicated as bell might instead be used as a control shift; also, in addition to the upper rail and lower rail shown, another option providing more of the ASCII character set might be seen as desirable.

That could lead to something like this:

In the simplest model, the three statuses of letters shift versus figures shift, unshift versus shift, and lower rail versus upper rail would be independent and static. A more complex model might promote greater transmission efficiency. Thus, there could be one independent static status of shift versus unshift that applies while in letters shift, while entering figures shift would always start from the more common unshifted state. Upper rail versus lower rail would remain static and independent as a status, but the shift codes for them, only available in figures shift, could also change from unshifted to shifted, since it is only in that status that the distinction is visible, and so setting the upper rail/lower rail status could also serve as an indication that moving to shifted status, where characters the printing of which is affected by it, is desired.

Integration Into a Larger Whole

The different schemes of extending ITA 2 above each have their advantages and drawbacks. And, of course, different character sets are also required for different purposes. If some special shift code combinations, whose use would not conflict with those used in repertoires of types I through V were used to select a repertoire and a language, one could have an extended ITA 2 that could be used to access the character sets for any number of languages, each one having a repertoire designed around it.

Of course, languages like Thai or Armenian would be served more efficiently by a 6-bit code than by a 5-bit code. They could still be handled, as Russian is, by placing the least frequent letters in the supplementary alphabetic set. But languages such as Chinese could be efficiently served, for example by using pairs of letters to represent a character in the normal letters shift, and sets of three letters to represent a character after an ALT character indicates the supplementary character set is to be used.

For the purpose of switching between repertoires at a high level, I propose the sequences ALT LTRS FIGS LTRS and FIGS LTRS FIGS LTRS. In this way, many shorter sequences are available, allowing additional functions to be incorporated into an extended version of the type V character repertoire which will be described below. The additional possibilities would also, of course, allow extension to character repertoires of the four other types if desired.

Further Elaboration of Type V

To allow a Type V repertoire to include many different alphabets and special character sets, I had proceeded to define a large number of shift code combinations, as follows:

[LTRS]
Switches into the default case of the current alphabetic character set. When superfluous, toggles between upper and lower case, and, if the default case is upper, changes it to lower.
[FIGS]
Switches into the standard set of digits and punctuation marks. When superfluous, switches to printing characters from the current set of extended figures-shift characters.
[ALT]
Causes the next character only to print as a member of the current set of extended characters; extended punctuation marks if in FIGS case, or extended alphabetic characters if in LTRS case. When in the case reached by a superfluous FIGS character within figures shift, that is, when printing extended figures-shift characters, case or when in ALT FIGS LTRS case, that is, when printing extended letters-shift characters, is treated as superfluous, and toggles or cycles between different sets of extended punctuation marks or different sets of extended alphabetic characters as the case may be, if multiple sets of these extended characters exist. It does not cause a return to printing normal punctuation marks or normal alphabetic characters, as that is done using the FIGS and LTRS shifts respectively.
[FIGS][LTRS]
Switches into the upper case of the current alphabetic character set, and makes upper case the default case. This provides a fixed point to the cycle caused by superfluous LTRS characters in LTRS case.
[ALT][LTRS]
Toggles between two national scripts, or cycles between multiple national scripts. Also resets the set of extended punctuation marks in use to the first available one within the new script, and the set of extended alphabetic characters in use to the first available one within the new script. As it is never superfluous, it does not cause a transition between upper and lower case when used when printing letters; when encountered while printing figures, it causes a return to the default case for the current mode, as it does cause entry to letters shift, and upon such entry causes standard rather than extended alphabetic characters to be printed.
[ALT][FIGS][LTRS]
Returns to the first national script for the current repertoire. This provides a fixed point to the cycle caused by the ALT LTRS sequence. Also resets the set of extended punctuation marks in use to the first available one within the new script, and the set of extended alphabetic characters in use to the first available one within the new script. It does not cause a transition between upper and lower case when used when printing letters; when encountered while printing figures, it causes a return to the default case for the current mode, as it does cause entry to letters shift, and upon such entry causes standard rather than extended alphabetic characters to be printed.
[ALT][ALT]G
Advances to using the next national script currently available. It resets the sets of extended punctuation marks and extended alphabetic characters in use to the first ones, and switches to printing standard rather than extended characters in both figures and letters shifts. It does not affect whether one is in figures or letters shift or in upper or lower case.
[ALT][ALT]H
Selects the first available national script. It resets the sets of extended punctuation marks and extended alphabetic characters in use to the first ones, and switches to printing standard rather than extended characters in both figures and letters shifts. It does not affect whether one is in figures or letters shift or in upper or lower case.
[ALT][ALT][LTRS]
Switches to printing extended alphabetic characters. To toggle between upper and lower case in this mode, a superfluous ALT ALT LTRS sequence is required; ALT advances to a different set of extended alphabetic characters, and LTRS returns to printing standard alphabetic characters.
[ALT][ALT]E
If encountered in letters shift, switches to printing extended alphabetic characters; if encountered in figures shift, switches to printing extended punctuation marks.
[ALT][FIGS]
Advances to the next set of extended punctuation marks, but does not cause a shift to printing them. If encountered in letters shift, it will cause a shift to printing normal figures shift characters. Note that if encountered when printing extended punctuation marks, its function would be identical to that of the ALT character by itself; it could be treated as superfluous, and given a special function in that case.
[LTRS][FIGS]
Switches to printing the default set of extended punctuation marks. This provides a fixed point to the cycle whose utilization is made available either by superfluous ALT characters in the case entered by a superflous FIGS shift, or by the ALT FIGS sequence.
[ALT][LTRS][FIGS]
Switches to using the default set of extended punctuation marks, but only switches to printing the default set of figures shift characters. This provides a fixed point to the cycle whose utilization is made available either by superfluous ALT characters in the case entered by a superflous FIGS shift, or by the ALT FIGS sequence.
[LTRS][FIGS][LTRS]
Switches to the default set of extended alphabetic characters. This provides a fixed point to the cycle caused by superfluous ALT characters in ALT FIGS LTRS case.
[ALT][ALT]A
Advances to making the next currently available set of extended punctuation marks the one to be used when printing extended punctuation marks without either changing from letters shift to figures shift, or changing from standard punctuation marks to extended punctuation marks.
[ALT][ALT]B
Advances to making the next currently available set of extended alphabetic characters the one to be used when printing extended alphabetic characters without either changing from figures to letters shift, or changing from standard alphabetic characters to extended alphabetic characters.
[ALT][ALT]C
Sets the default set of extended punctuation marks as the extended punctuation mark set to use without either changing from letters shift to figures shift, or changing from standard punctuation marks to extended punctuation marks.
[ALT][ALT]D
Sets the default set of extended alphabetic characters as the extended alphabetic characters to use without either changing from figures to letters shift, or changing from standard alphabetic characters to extended alphabetic characters.
[ALT][ALT][FIGS]
This sequence, followed by a one-letter code, is used to select one of a number of character repertoires which are intended to differ primarily in their extended figures-shift characters, but which may also differ elsewhere, representing different applications areas.
[ALT][LTRS][FIGS][LTRS]
This sequence, followed by a code which may be one, two, or three characters in length, as determined by the national standards body for the country whose codes are being used, selects a coding system which may include multiple character repertoires reachable by the ALT FIGS escape, multiple languages reachable by ALT LTRS, or which may operate in a different fashion from that outlined here, for example by using ALT as a third shift.
[FIGS][LTRS][FIGS][LTRS]
This sequence, followed by a two-letter country code, switches into the letters shift of the default character repertoire for the given country, in upper case mode, and selects the family of character repertoires defined by that country's official standards body as that which subsequent shifts will reach.

State Diagrams

The following diagram may make this form of the proposal for character repertoires of type V a little clearer, although it only illustrates part of what it involves.

Only the codes that remain within a single character repertoire for a single language are depicted. Thus, what happens when a superfluous ALT code is found when printing extended figures-shift characters continuously (that is, within the mode reached by a superfluous FIGS code within figures-shift mode) and when printing extended letters-shift characters continuously (that is, within ALT ALT LTRS mode) is only shown as a small green arrow pointing off to one side; there is only an effect if there is more than one set of extended figure or alphabetic characters, respectively, and the result of an ALT LTRS code, which switches to another language entirely, is also not shown.

Upper case mode is shown in blue, normal lower case mode in black. The red arrows show how a superfluous LTRS code encountered in upper case mode, as well as a superfluous ALT ALT LTRS code, switches from upper case mode to lower case mode as well as to lower case, and how FIGS LTRS switches from lower case mode back to upper case.

Although it should be apparent what the different boxes in the diagrams stand for, making it explicit which modes they refer to may help in ensuring there is no ambiguity in the description of this proposed mode:

 --------------    --------------
 | upper-case |    | upper-case |
 | mode       |    | mode       |
 |            |    | extended   |
 | figures    |    | figures    |
 | shift      |    | shift      |
 |            |    |            |
 |            |    |            |
 --------------    --------------

 --------------
 | upper-case |  
 | mode       |         --------------    --------------
 |            |         | lower-case |    | lower-case |
 | letters    |         | mode       |    | mode       |
 | shift      |         |            |    | extended   |
 | upper      |         | figures    |    | figures    |
 | case       |         | shift      |    | shift      |
 --------------         |            |    |            |
                        |            |    |            |
 --------------         --------------    --------------
 | upper-case |
 | mode       |         --------------    --------------
 | extended   |         | lower-case |    | lower-case |
 | letters    |         | mode       |    | mode       |
 | shift      |         |            |    |            |
 | upper      |         | letters    |    | letters    |
 | case       |         | shift      |    | shift      |
 --------------         | upper      |    | lower      |
                        | case       |    | case       |
                        --------------    --------------

                        --------------    --------------
                        | lower-case |    | lower-case |
                        | mode       |    | mode       |
                        | extended   |    | extended   |
                        | letters    |    | letters    |
                        | shift      |    | shift      |
                        | upper      |    | lower      |
                        | case       |    | case       |
                        --------------    --------------

The diagram may also make explicit the substance of the different cases. Instead of using national-use positions in the figures case, any extra letters needed for a given language are placed in the set of extended alphabetic characters. The extended figures characters are intended to be usually used for the kinds of characters reached by the ALT character in ASCII over AMTOR; the other useful characters included in ASCII for which there was no room in the figures case of normal 5-level code.

The following diagram:

illustrates more fully what happens when superfluous ALT codes are encountered, by depicting the transition between character sets with different extended alphabetic characters or different extended figures characters.

LTRS FIGS and LTRS FIGS LTRS, in a sense, work the same way as a superfluous ALT code, except they only move to the first of the possible extended figures character sets or the first of the possible extended alphabetic character sets respectively, but they shift to those character sets from anywhere, preserving the distinction between upper-case mode and lower-case mode, and, in the case of LTRS FIGS LTRS, the distinction between upper and lower case themselves as well.

As for ALT LTRS, the following diagram attempts to show how it works:

Again, ALT FIGS LTRS, not shown, works the same way as ALT LTRS, except that it only causes a transition directly to the first of the available languages.

For reasons of legibility, it was only able to show the transition from one language to the next; it must also be recognized that ALT LTRS would cause the same transition from the second language in the diagram, either to a third one in a cycle, or back to the first one if there are only two languages to toggle between.

A Difficult Case

After having developed much of the scheme outlined above, I saw that it would work nicely enough with Egyptian hieroglyphics: that language has an alphabet of 22 or 24 letters plus a wide symbol repertoire which could be handled by two or three code symbols in the extended alphabetic set. One could use the case shift to cycle through three character sets, hieroglyphic, hieratic, and demotic, despite the fact that they're not strictly isomorphic in the way that upper-case and lower-case are; this would be a trivial and obvious extension of the scheme. Or those character sets could be treated as different languages.

But that led me to thinking of Japanese. If one uses case-shifting for non-isomorphic character sets, treating hiragana as the lower case of katakana naturally comes to mind. But both of those syllabaries have considerably more than 26 characters, and so one has to use the extended alphabetic characters for the less common syllables.

And exactly where does that leave kanji?

One way to address this is to make use of the fact that it is normally the hiragana characters, not the katakana characters, that are used with kanji. Hence, a reasonably efficient route exists in the framework of the scheme as outlined, as follows:

Use the superfluous LTRS code to switch between kanji and hiragana, and use ALT LTRS to switch between this mode and katakana.

A particularly elegant possibility is to begin in katakana mode, switch to kanji with the first superfluous LTRS, toggle between kanji and hiragana with any subsequent superfluous LTRS, and return to katakana using the FIGS LTRS sequence. This makes kanji the case to which one returns when using LTRS to return from printing figures characters: katakana is the "original" upper case, kanji is lower case, and hiragana is the upper case used in lower case mode. By avoiding the use of ALT LTRS, this causes Japanese to be treated as a single language, which, of course, it is.

A more pedestrian route would be to use ALT LTRS to cycle between the Latin alphabet and katakana and hiragana, with the Latin alphabet being the base character set reached by ALT FIGS LTRS, and to access kanji by the use of a superfluous ALT code when in the ALT ALT LTRS case of the hiragana character set and likely also the katakana character set. This avoids the use of upper and lower case for tasks for which they are not really intended.

Restrictions on the Codes

Given that the sequence FIGS LTRS is determined by Recommendation S. 2, and ALT is the only available single-character code, and thus is needed for the operations requiring the shortest codes, how is it possible to choose codes that can appear consecutively, without ambiguity, and still have a wide selection of relatively short codes?

One troublesome situation that I noted after assigning many of the shift sequences to my extended version of the type V character repertoire is that it might be desired, after shifting to a particular script, to print as the first character in that script a character from its extended alphabetic set, or to switch from upper to lower case or the reverse at the same time as one switches scripts.

To allow as many short codes as possible, an advantage can be derived by taking into account how the codes are used. For compatibility, any code ending in LTRS would select some type of alphabetic characters, and any code ending in FIGS would select some type of special characters. Thus, a superfluous LTRS code might be used immediately following a code ending in LTRS, but a FIGS code would not be needed immediately after such a code. But ALT is used as an escape in either case.

Thus, I initially decided it would be sufficient to restrict codes to the following form:

In most cases, this leads to no problem, but there are occasional cases in which it might be desired to have two codes following each other in a way that would create ambiguity; for example, ALT LTRS, used to switch to another national script, cannot be followed immediately by FIGS, even though there might be a change in the figures shift when changing scripts as well. This is, however, unlikely. ALT LTRS is being used the way ALT had been used as a third shift; changing from using the Latin alphabet to using the alphabet of a national script within a character repertoire associated with a given nation. So it is intended that the figures shift will not change; this is further facilitated by the fact that extended alphabetic characters, associated with a given alphabet, have their own place, and need not be included among figures shift characters.

The possibility that such problems may arise, however, was addressed by using ALT ALT as an escape to create a series of shift codes. As these codes contain neither LTRS nor FIGS, they do not cause a switch between letters and figures shift, but they can switch which set of printable characters is to be used in other respects, and thus they are used for mode changes which may need to be combined with other mode changes in a flexible manner.

Tabular Form

The following table summarizes the actions of the various shift codes which operate within this particular type of character repertoire:

Code LETTERS FIGURES shift UPPER LOWER case UPPERCASE LOWERCASE mode extended figures shift character set extended letters shift character set NORMAL EXTENDED figures shift characters NORMAL EXTENDED letters shift characters national script
LTRS if in figures shift or if printing extended letters shift characters letters sets to match mode nc nc nc normal normal nc
FIGS if in letters shift or if printing extended figures shift characters figures nc nc nc nc normal normal nc
LTRS if printing normal letters shift characters letters (nc) inverts lowercase nc nc nc nc nc
ALT ALT LTRS if printing extended letters shift characters letters (nc) inverts lowercase nc nc nc nc nc
FIGS LTRS letters upper uppercase nc nc nc nc nc
FIGS if printing normal figures shift characters figures (nc) nc nc nc nc extended nc nc
ALT ALT LTRS if not printing extended letters shift characters letters sets to match mode if not in letters shift nc nc nc nc extended nc
ALT ALT E nc nc nc nc nc extended if in figures shift extended if in letters shift nc
ALT if printing extended characters nc nc nc increments if in figures shift increments if in letters shift nc nc nc
ALT FIGS figures nc nc increments nc nc nc nc
LTRS FIGS figures nc nc first nc extended nc nc
ALT LTRS FIGS figures nc nc first nc nc nc nc
LTRS FIGS LTRS letters sets to match mode if not in letters shift nc nc first nc extended nc
ALT ALT A nc nc nc increments nc nc nc nc
ALT ALT B nc nc nc nc increments nc nc nc
ALT ALT C nc nc nc first nc nc nc nc
ALT ALT D nc nc nc nc first nc nc nc
ALT LTRS letters sets to match mode if not in letters shift nc first first normal normal increments
ALT FIGS LTRS letters sets to match mode if not in letters shift nc first first normal normal first
ALT ALT G nc nc nc first first normal normal increments
ALT ALT H nc nc nc first first normal normal first

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