I've heard of the Mongolian vowel separator from programmers, who regard it as an interesting quirk in Unicode. When I google for it, most of the hits are from those revelling in its geekiness.
But as someone dabbling a bit in Mongolian, I'd like to know: What is it used for?
The formal description has been already given in the excellent @ColinFine's answer. Let me give a different description in "layman terms".
Mongolian characters usually have four distinct forms: isolate, initial, medial, and final.
Vowels A and E have exactly the same glyphs in their final form.
Here are the four forms for A and E, correspondingly.
Note, both have two versions of final glyphs:
Although choosing between A or E can be concluded from the the syntax (A for masculine grammatic gender while E is for feminine), there can be semantic difference depending on the final form (stroke up or stroke down).
For example:
[qara] (to look), stroke up;
[qar+a] (black), stroke down;
The Vowel Separator is used in the second word.
Phonetically, there's a little pause before the final vowel.
Note that A does not obtain the isolate form. Instead, it only changes to the second final form. Also, R gets the final form in the second word.
As per why the character makes programmers' hell (just in case if you wonder).
Most of the modern-day compilers allows Unicode identifiers (e.g., variable names). You may write your program with variables in your (non-English) language, and your program works just fine.
However, using U+180E may lead you into a trouble because it may or may not be considered a symbol. Here's what happens (assume that X is the Mongolian Vowel Separator):
integer variable aXa = 42;
print aa;
Note: since the X symbol is invisible, the first line on your screen looks like:
integer variable aa = 42;
Trouble one: Unicode versions prior to 4.0 treat X as a formatting (thus, valid) character, but trying to use the variable aa leads you to an error because there is no such variable! There's only aXa, but you can't see it.
Trouble two: Unicode 4.0 treats X as a zero-width space. This means that you're trying to declare a variable with space in its name (a a) which makes your code unable to compile. But again, due to invisibility you simply don't know what's wrong. The code visually looks perfectly valid.
References:
The Unicode 30 spec says, in section 11.4:
U+180E MONGOLIAN VOWEL SEPARATOR is a word-internal thin whitespace that may occur only before the word-final vowels U+1820 MONGOLIAN LETTER A and U+1821 MONGOLIAN LETTER E. It determines the specific form of the character preceding it, selects a special variant shape of these vowels, and produces a small gap within the word..."
The tables of Mongolian letters in both Bright and Daniels The World's Writing Systems (ISBN 0-19-507993-0) and Campbell's Concise Compendium of the World's Languages (ISBN 0-415-11392-X) show distinct forms for vowels A and E only initially, so the issue of distinguishing A and E doesn't seem to arise at the end of the word. The sample glyphs for U+1820 and U+1821 in the printed Unicode standard do not seem to me to match closely any of the glyphs shown in either of those sources.
However the sample of Mongolian in Bright and Daniels (an 18thC blockprint) does show a number of words ending in a glyph which in Campbell's table is the final form of A/E (confusingly, that particular glyph does not appear at all in the accompanying table in Bright and Daniels); and one word (which occurs three times) has this final glyph doubled, with a tiny space between. Normally one would expect that the penultimate letter would take a medial form, so I'm guessing that this is the purpose for which 180E was introduced.
I don't know Mongolian, but I'll venture a guess. There is front/back vowel harmony in Mongolian, so a distinction in writing between word final "a" and "e" is not all that necessary. It will be /a/ in back harmonic words and /e/ otherwise. The vowel separator glyph seems to have been pressed into service to make this distinction for the sake of non-human readers.
I found some discussion and illustrations here on pages 5-6.
