U+0308 COMBINING DIAERESIS

U+0308 was added to Unicode in version 1.1. It belongs to the block Combining Diacritical Marks in the Basic Multilingual Plane.

This character is a Nonspacing Mark and inherits its script property from the preceding character. The character is also known as double dot above, umlaut, Greek dialytika and double derivative.

The glyph is not a composition. It has a Ambiguous East Asian Width. In bidirectional context it acts as Nonspacing Mark and is not mirrored. The glyph can, under circumstances, be confused with 1 other glyphs. In text U+0308 behaves as Combining Mark regarding line breaks. It has type Extend for sentence and Extend for word breaks. The Grapheme Cluster Break is Extend.

The Wikipedia has the following information about this codepoint:

The diaeresis and the umlaut are diacritics that consist of two dots ( ¨ ) placed over a letter, most commonly a vowel. When that letter is an i or a j, the diacritic replaces the tittle: ï.

The diaeresis is used to denote the phonological phenomenon also known as diaeresis ( /daɪˈɛrɨsɨs/ dy-ERR-ə-səs), in which a vowel letter is not part of a digraph or diphthong. The umlaut mark ( /ˈʊmlaʊt/ UUM-lowt) denotes a sound shift. The two uses originated separately, with the diaeresis being considerably older. In modern computer systems using Unicode, the umlaut and diaeresis diacritics are identical: ⟨ä⟩ represents both a-umlaut and a-diaeresis. Two dots are also used as a diacritic in other cases, where they are neither diaeresis nor umlaut.

Diaeresis

The diaeresis indicates that two adjoining letters that would normally form a digraph and be pronounced as one are instead to be read as separate, either as a diphthong or as two distinct vowels in two syllables. The diaeresis indicates that a vowel should be pronounced apart from the letter that precedes it. For example, in the spelling coöperate, the diaeresis reminds the reader that the word has four syllables co-op-er-ate, not three, *coop-er-ate. In British English this usage has been obsolete for many years, and in US English, although it persisted for longer, it is also now considered archaic. Nevertheless, it is still used by the U.S. magazine The New Yorker. However, languages such as Dutch, Catalan, French, and Spanish make regular use of the diaeresis.

Umlaut

"Umlaut" refers to a historical sound shift in German. In German, umlauts are found as ä, ö and ü. The name is used in some other languages that share these symbols with German or where the Latin spelling was introduced in the 19th century, replacing marks that had been used previously. The phonological phenomenon of umlaut occurred historically in English as well (man ~ men; full ~ fill; goose ~ geese) in a way cognately parallel with German, but English orthography does not write the sound shift using the umlaut diacritic. Instead, a different letter is used.

The umlaut diacritic originated from German Kurrentschrift, of which the Sütterlin script is an example, a type of cursive script previously widely used in German speaking countries, in which the small letter "e" looked very different from typical forms in English and other cursive scripts. This is explained in more detail in the German article: [1]. A Kurrent "e" is written as two short vertical strokes, very close together, linked to the preceding and/or following letters.

Other

By extension, the double dot diacritic is also used to denote similar distinctions, such as marking the schwa ë in Albanian.

In the International Phonetic Alphabet, a double dot 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.