Diacritical marks of two dots , placed side-by-side over or under a letter, are used in a number of languages for several different purposes. The most familiar to English-language speakers are the diaeresis and the umlaut, though there are numerous others. For example, in Albanian, represents a schwa. Such dots are also sometimes used for stylistic reasons (as in the family name Brontë or the band name Mötley Crüe).
In modern computer systems using Unicode, the two-dot diacritics are almost always encoded identically, having the same code point. For example, represents both a-umlaut and a-diaeresis. Their appearance in print or on screen may vary between typefaces but rarely within the same typeface.
Uses
Diaeresis
The "diaeresis" diacritic is used to mark the separation of two distinct vowels in adjacent syllables when an instance of diaeresis (or hiatus) occurs, so as to distinguish from a digraph or diphthong. For example, in the spelling "coöperate", the diaeresis reminds the reader that the word has four syllables co-op-er-ate, not three. It is used in several languages of western and southern Europe, though rarely now in English.
Umlaut
The "umlaut" diacritic indicates a sound shift phenomenon also known as umlaut in which a back vowel becomes a front vowel. It is a specific phenomenon in German and other Germanic languages, affecting the graphemes , , and , which are modified to , , and .
Stylistic use
The two dot diacritic is also sometimes used for purely stylistic reasons. For example, the Brontë family, whose surname was derived from gaelic and had been anglicised as "Prunty", or "Brunty": At some point, the father of the sisters, Patrick Brontë (born Brunty), decided on the alternative spelling with a diaeresis diacritic over the terminal to indicate that the name had two syllables.
Similarly the "metal umlaut" is a diacritic that is sometimes used gratuitously or decoratively over letters in the names of hard rock or heavy metal bandsfor example, those of Motörhead and Mötley Crüe, and of parody bands, such as Spın̈al Tap.
Other uses by language
A double dot is also used as a diacritic in cases where it functions as neither a diaeresis nor an umlaut. In the International Phonetic Alphabet (IPA), a double dot above a letter is used for a centralized vowel, a situation more similar to umlaut than to diaeresis. In other languages it is used for vowel length, nasalization, tone, and various other uses where diaeresis or umlaut was available typographically. The IPA uses a double dot below a letter to indicate breathy (murmured) voice.
Vowels
In Albanian, Tagalog, and Kashubian, represents a schwa [ə]. In Aymara, a double dot is used on for vowel length. In the Basque dialect of Soule, represents In the DMG romanization of Tunisian Arabic, , , , , and represent , , , , and . In Ligurian official orthography, is used to represent the sound . In Māori, a diaeresis (e.g. ) was often used on computers in the past instead of the macron to indicate long vowels, as the diaeresis was relatively easy to produce on many systems, and the macron difficult or impossible. In Seneca, are nasal vowels, though is , as in German umlaut. In Vurës (Vanuatu), and encode respectively and . In the Pahawh Hmong script, a double dot is used as one of several tone marks. The double dot was used in the early Cyrillic alphabet, which was used to write Old Church Slavonic. The modern Cyrillic Belarusian and Russian alphabets include the letter (yo), although replacing it with the letter without the diacritic is allowed in Russian. Since the 1870s, , (Cyrillic letter yi) has been used in the Ukrainian alphabet for iotated ; plain і is not iotated . In Udmurt, ӥ is used for uniotated , with и for iotated . The form is common in Dutch handwriting and also occasionally used in printed text – but is a form of the digraph "ij" rather than a modification of the letter . Komi and Udmurt use (a Cyrillic O with two dots) for . The Swedish, Finnish and Estonian languages use and to represent and In the languages of J.R.R. Tolkien's Middle-Earth novels, a diaeresis is used to separate vowels belonging to different syllables (e.g. in Eärendil) and on final e to mark it as not a schwa (e.g. in Manwë, Aulë, Oromë, etc.). (There is no schwa in these languages but Tolkien wanted to make sure that readers wouldn't mistakenly pronounce one when speaking the names aloud.)
Consonants
Jacaltec (a Mayan language) and Malagasy are among the very few languages with a double dot on the letter "n"; in both, n̈ is the velar nasal .
In Udmurt, a double dot is also used with the consonant letters ӝ (from ж ), ӟ (from з ) and ӵ (from ч ).