Digraphs, ligatures, dipthongs
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Ligatures and digraphs
A ligature joins two characters into a graphically different form. This includes:
- alternatives such as æ as ae - which may be purely stylistic, or may be semantically different.
- typographical. From the days of metal movable type, something with overhang such as f (a particular problem case) followed by something like an i (or anything else) would look widely spaced. Compare 'fi' with 'ﬁ'. Even with computed kerning, there is cleanliness to consider. Making fi overlap means the dot on the i and the f's curl may overlap - the pair still needs to be treated as a special case. (See also  under the ligatures heading)
- visually nontrivial orthography. Outside the orthographical rules, this has little or no semantical implications. Interesting examples include various Indic scripts, which may even represent one character with multiple glyphs (e.g. one vowel with separate graphemes).
Additional orthographical rules may apply to all ligatures, such as when abbreviating or hyphenating them.
Digraphs (also known as bigraphs and digrams) write a single sound as two characters (that lose that meaning when separated) because a single character for that sound does not exist, is archaic, or not convenient.
Trigraphs and tetragraphs also exist.
Note this deals with sound rather than type or orthography. Some ligatures are also digraphs, some are not. Non-typographical ligatures are often digraph ligatures.
Non-ligature digraphs include sounds that are written as several graphemes, often because there is no specific grapheme for it. For example:
- 'ch' in English, such as in 'school.' (It is not an allophone of any reduced form or anythings else)
- dipthongs are essentially vowel-only digraphs. That is, they graphically represent dipthongs sounds.
Digraph ligatures may be phonetically identical spelling variants. In German, ä and ae, ö and oe, ü and ue, and to a slightly lesser extent ß and ss, are equivalents that largely became such because of systems that did not support them like Morse and typewriters(verify). This also caused diaresis and umlaut variations to be joined into the same visual letter.
Note that conversibility depends on language and context. German appearances of ue are only be seen as a ü if they were a single sound, and not e.g. in aktuelle.
Digraph ligatures may also grow to be considered a separate sound, sometimes even have alphabet status. Swedish and Finnish have Å, Ä, and Ö, Norwegian and Danish have Æ, Ø, and Å. Some have unambiguous separated forms, some do not.
In English, there is no particular difference between, for example, encyclopædia, encyclopaedia, both now considered archaic in favour of encyclopedia.
The English w is another historical example, as it originated as 'vv', two Latin Us (which is why its name is double-u). W replaced the Ƿ character (Wynn) in Old English.
Ligatured combinations with a specific function are sometimes accepted as symbols - technically becoming logograms. Consider & which came from a ligature of the Latin 'et', and the dollar sign $.