APL Under-bar Characters

Ken Whistler kenwhistler at att.net
Tue Aug 18 11:22:53 CDT 2015

Returning to a historical note on the glyphic forms and the question
of combining low lines or combining macrons below... admittedly a
side note on this thread, the *original* identification of these APL
uppercase Latin letters, at least in their IBM implementations, was
clearly as uppercase (italicized) Latin letters with *underscores* --
not with macrons below.

The identification of the entity we have been talking about, for example,
in IBM documentation is *LA480000*, described as:

"A Line Below Capital/A Underscore (APL)"

It is shown in the documentation with an *underscore*, with the
scoring reaching to match the outside serifs of the "A", and clearly
not with a macron below.

Furthermore, the glyph character identification system from that era
(late 1980's) contained a value for a "Line below" diacritic (that's the
"48" in the glyph identifier above), but no provision for macrons
*below* a letter.

Consider also that the keyboards and character sets involved in
the time had underscores (low lines), but macrons below were
rare diacritics, and not in anybody's character set at the time.

The appearance of underscored characters in printed material
at the time would typically involve a gap between the
underscoring on adjacent characters, but that was a result of
discrete type elements in the print trains, typically. It wasn't because
conceptually the underscores were being treated as deliberately short
diacritics that should *not* connect. The underscores were more
likely to connect on screens, but that was typically the result
of the very limited scale of the character generator pixel
rasters for the characters. You just turned on all the pixels in
the bottom row of the box -- and there you had your underscore!

The documentation that Doug cites from Section 7.9 of TUS
was written to clarify that the *general* intent, when people
use underscoring or overscoring diacritics, is that they should
connect laterally. That is to contrast with macron diacritics, above *or*
below, which of course do not connect laterally to adjacent
macrons. But without a very specialized font, it is very difficult
to do lateral connection correctly for variable width fonts. See 
examples below
for Helvetica, Times, and Courier (although your mileage
may differ, depending on your email client fonts, as Doug noted):




Sequences of combining low lines after letters on the left, styling
with underscoring on the right. Only for fixed width fonts does
this really work "as designed", so to speak. Hence, the general
recommendation that if what you are trying to do is underscore
(or overscore) a sequence of text, by all means do it with styling,
and not with sequences of individual diacritics on letters.

But the fact that underscores used as diacritics on letters are
basically a 20th century typewriter hack that persisted into
early computer character sets -- and the fact that they don't
work very well, or look very elegant with most modern,
digital, variable width fonts, interestingly has led to the rise of the 
below, very much along the lines of Doug's suggestion cited below.
What used to be a rare diacritic is gaining in popularity in
actual use, precisely because it "looks like" a diacritic on
the letter in most fonts nowadays, and because it *doesn't*
connect, more or less randomly, with neighboring diacritics
on adjacent letters, the way the low line diacritic can.

And while I am generally sympathetic with this changeover, and
suspect it is probably the best outcome for cases like the use of
line below diacritics on Latin letters in Semitic transliteration,
I don't think it is the best recommendation for this particular
case of a legacy usage for APL. The APL letters with underscores
clearly *are* historically connected precisely to the underscore,
and should probably stay represented accordingly. If APL
afficionados don't prefer the underscores visually connecting between
adjacent capital Latin letters in APL text material presented that
way, then that can be addressed in the APL specialist fonts. After
all, such fonts already exist, precisely to provide best display
for all the other specialized symbols of APL.


On 8/17/2015 9:23 AM, Doug Ewell wrote:
> TUS 7.0, Section 7.9 does say:
>> are intended to connect on the left and right.
> In that case, despite the text in Section 22.7 that Ken quoted, it seems
> that U+0331 COMBINING MACRON [BELOW] might be a better choice for APL
> "underlined letters" than U+0332 COMBINING LOW LINE. Compare A̱ḆC̱
> with A̲B̲C̲, noting that your font and rendering engine mileage may
> vary.

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