The
glyph ä, known as
U+00E4 LATIN SMALL LETTER A WITH DIAERESIS in
Unicode and
ä in
HTML, denotes at least three different
sounds and is found in a number of primarily
European scripts.
The most common use of "ä" is in German and Swedish
as a representation for umlaut. To briefly summarize, in some
transformations (esp. plurals) in these languages the vowel sound
moves from back to front,
as in German Mann becoming Männer, where the "ä" is
pronounced the same as the "e" in English "men". However,
since the sound was originally written by adding a small e atop the
letter a, many Germanic languages (including Danish,
Norwegian and Icelandic) have opted to use the
AE ligature æ instead; some other Germanic languages, like
English and Dutch, just use a simple e. (Yes, English man/men
is the same case of umlaut, spelled a little differently.)
But when the Swedes hopped across the Gulf of Finland and conquered
the illiterate peasants on the other side, things started
to get a bit hairy. Finnish is not a Germanic language,
and it differentiates not only between back vowels and front vowels,
but open vowels and closed vowels as well. This distinction
was not found in the Swedish script, so at first "e" and "ä" were used
rather indiscriminately. Eventually this evolved into using "e"
for the closed vowel (English "men", IPA e) and "ä" for the
open variant (English "bad", IPA æ1).
This selection was intuitive, since instead of umlaut Finnish
has vowel harmony: whereas in German "a" tends to change into "ä",
in Finnish conjugations change so that "a" and "ä" are never found in the
same word.
Finnish orthography finally being settled, it was
borrowed into most other languages in the Finnish branch of the
Finno-Ugrian family, including Estonian and Sámi.
Hungarian makes do without though, since the sound denoted by "ä"
is not found in the language, and they worked out
their own orthography anyway. The letter "ä" is also employed in some Turkic languages, although
Azeri has recently (and inexplicably) decided to
start using the IPA schwa sign instead.
Finally, while "ä" could theoretically be used for diaeresis even in
English (as in coöperation; see the node for the
full scoop), this turns out to be quite rare in practice.
According to Gritchka, our resident master of obscure
linguistic trivia, some poetry by Barnes and
Tennyson in the English West Country dialects does use the diaeresis
over a, as does Tolkien's Elvish for the initial
combinations eä and eö.
1: Yup, the same æ that was used for "e" in
the last paragraph; the Danish æ is not the same as the
German and happens to match the Finnish ä. Confused yet?
References
http://www.unicode.org
http://www2.arts.gla.ac.uk/IPA/vowels.html
http://www.lysator.liu.se/language/Languages/Swedish/Grammar.html
http://www.internetix.ofw.fi/opinnot/opintojaksot/8kieletkirjallisuus/aidinkieli/kirjasuomi/konsvok.html