The letters of an alphabet are written symbols for the smallest meaning-distinguishing phonetic units of language, the phonemes; for example, ⟨t⟩ and ⟨s⟩ in ⟨skin⟩ and ⟨house⟩ distinguish the meaning of words (see also minimal pair and allophone).
In an ideal alphabet, each letter corresponds to a phoneme and vice versa. In practice, however, deviations are always found:
- The same sign may apply to different sounds (e.g. ⟨v⟩ for [f] in bird and [v] in vase, or the three ⟨e⟩ in give away [ˈvɛkˌgeːbən]).
- The same sound can be notated with different characters (e.g. [f] in ⟨Bird⟩ and ⟨Fish⟩).
- Several characters can stand for a single phoneme (⟨sch⟩).
- Multiple sounds can be represented by a single character (e.g. ⟨x⟩ for /ks/).
- A sound may be left unsigned (e.g. the glottal stop in ⟨beachten⟩ /bəˈʔaxtən/).
Moreover, the correspondence of phoneme and grapheme, once established, is also lost through language change (compare English ⟨sign⟩ /saɪn/ and ⟨signal⟩ /ˈsɪgnəl/ versus Latin ⟨signum⟩).
If a writing system lacks signs for phonemes, linguistic (content-related) differences may not be reproduced in writing. For example, some alphabets originally consisted only of consonants (consonant writing). Later, they were supplemented with signs for vowels, which could be placed as small additions (e.g. dots, dashes) to the consonants (e.g. Arabic and Hebrew alphabets).
If, on the other hand, signs for phonemes are present in abundance in a writing system, semantic (content-related) differences can be expressed in writing even if the phonemes are the same. For example, in German ⟨Lerche⟩ and ⟨Lärche⟩.
The writing systems for most European languages use variants of the Latin alphabet. Similar sounds of the respective language were assigned to the characters for Latin sounds. The same characters stood for partially different sounds in the different languages. In addition, further changes in pronunciation have occurred in the course of language development (cf. ⟨j⟩ in German and English).
Since the number and type of phonemes differ in the various languages, the character set of the Latin alphabet was often not sufficient. Therefore, letter combinations (e.g. ⟨ou⟩, ⟨ch⟩, ⟨sz⟩) and diacritical marks (e.g. in ⟨ö⟩, ⟨š⟩) were introduced to represent the phonemes in question.
In addition, variants of the original Latin characters (⟨i⟩ > ⟨j⟩, ⟨v⟩ > ⟨u⟩) and ligatures (⟨ae⟩ > ⟨æ⟩, ⟨uu⟩/⟨vv⟩ > ⟨w⟩, ⟨ſz⟩/⟨ſs⟩ > ⟨ß⟩) evolved into characters in their own right, occasionally adopting letters from other alphabets (⟨þ⟩).
