Diaeresis
In linguistics, a diaeresis or dieresis (AE) (from Greek diairein, "to divide") is the modification of a syllable by distinctly pronouncing one of its vowels. The diacritic mark composed of two small dots placed over a vowel (¨) to indicate this modification is also called a diaeresis.
Usage
In French, Greek, and Dutch, and in English borrowings from them, this is often done to indicate that the second of a pair of vowels is to be pronounced as a separate vowel rather than being treated as silent or as part of a diphthong, as in the word naïve or the name Zoë. French also uses diaeresis over "i" to indicate syllabification in, for example, Gaëlle and païen. It is called trema or deelteken in Dutch, tréma in French.
The diaeresis is also occasionally used on native English words for the above purposes (as in "coöperate", "reënact", and the surname "Brontë"), but this usage has become very rare since the 1940s. The New Yorker magazine is noted as one of the few sources that still spells "coöperate" with a diaeresis.
In Spanish and Portuguese, it is used over the vowel u to indicate that it is pronounced in places where that vowel would normally be silent. In particular, the u is silent in the letter combinations gue and gui, but in words such as vergüenza ("shame") or pingüino ("penguin"), the u is in fact pronounced, forming a diphthong with the following vowel ([we] and [wi] respectively).
In Catalan, diaereses serve two different purposes. Similarly to Spanish, they are used in the groups güe, güi, qüe, and qüi to indicate that the u is in fact pronounced forming a diphthong with the following vowel ([we] and [wi] respectivelly). For example, aigües ("waters"), qüestió ("matter"). Also, similarly to French, diaereses are used over i or u to indicate that they do not form a diphthong with a preceding vowel. For example, veïna [b@'in@] ("neighbour", feminine), diürn [di'urn] ("diurnal").
Y-diaeresis can also be used in transcribed Greek: there it represents the non-diphthong αυ (alpha upsilon), e. g. in the Persian name Artaÿctes at the very end of Herodotus.
Similar looks, different functions
Umlaut
The similar diacritic mark is used for a different purpose in German: in this language it marks a variation in the pronunciation of vowels known as umlaut. Although sometimes rendered as two vertical or oblique bars above the letter, in most typescripts it is indistinguishable from diaeresis. The mark evolved from the ligatures æ and œ to a small 'e' written above the letter, simplified to bars or dots; the umlauts, when needed, can be substituted by 'ae', 'oe' and 'ue'.
Other evolved ligatures
In Finnish, Hungarian and North Germanic languages (i.e., Danish, Icelandic, Norwegian and Swedish) there are characters that appear similar to German umlauts (ü, ä, and ö), and represent sounds similar to the corresponding sounds in German. Despite this, they are in fact considered as letters in their own right, as is å. This is the reason why, unlike in German, it is not correct to replace them with 'ae' or 'oe'. The umlaut, particularly on the letter u, is also used in the transcription of languages that do not use the Roman alphabet, such as Chinese. For example, 女 (meaning "woman") is transcribed as nü.
As such uses do not mark grammatical variation, i.e. of tempus or modus, nor syllable modification, they are not properly cases of umlaut nor of diacritical marking. Hence it ought to be improper to call these characters umlauts; however, there is no better name to use in English.
The Dutch Y IJ/ij is sometimes written Ÿ/ÿ, but this is not a standard use. If the real IJ/ij characters are not available IJ/ij should be used. Ÿ is used because the Dutch Y represents a single letter in all cases, for example IJsselmeer.
How to produce the characters on computers
Using the ISO-8859-1 character encoding, one can type the letters ä, ë, ï, ö, ü, and ÿ. Dozens more letters with the diaeresis are available in Unicode. Unicode also provides the diaeresis as a combining character U+0308.
Despite the difference between diaeresis and umlauts, the HTML entities for these characters all end in uml; e.g. ä = ä.
TeX also allows double dots to be placed over letters in math mode, using "\ddot{}", or outside of math mode, with the \" control sequence:
