A digraph or bigraph is a pair of letters used to write one sound. This is often, but not necessarily, a sound (or more precisely a phoneme) which cannot be expressed using a single letter in the alphabet used for writing.

Sometimes, when digraphs do not represent a new phoneme, they are a relic from an earlier period in the language's history when they did (or remain phonemic only in certain dialects, e.g. wh in English).

Transliteration makes extensive use of digraphs.

There are three kinds of digraphs: sequences, reversals (really a special kind of sequence) and doubled letters.

Sequences

This is a group of two letters, both of which are different.

Examples from languages include:

• Basque
  • tx, corresponds to (voiceless postalveolar affricate)

• Czech
  • ch, corresponds to [x] (voiceless velar fricative)

• English
  • ch, usually corresponds to [tʃ] (voiceless postalveolar affricate) or [ʃ] (voiceless postalveolar fricative)
  • th, usually corresponds to [θ] (voiceless interdental fricative) or [ð], (voiced interdental fricative)
  • sh, corresponds to [ʃ], (voiceless postalveolar fricative)
  • ng, corresponds to [ŋ] (velar nasal)
  • kn, corresponds to [n] (alveolar nasal)
  • ph, corresponds to [f] (voiceless labiodental fricative)
  • ck, corresponds to [k] (voiceless velar plosive)
  • ea, ie, ei correspond mostly to [i] (close front unrounded vowel)
  • ai, ay correspond mostly to [e] (close-mid front unrounded vowel)
  • ue corresponds to [u] (close back unrounded vowel)

• French
  • ai, equivalent to è, corresponds to [ɛ] (open-mid front unrounded vowel)
  • au, corresponds to [o] (close-mid back rounded vowel)
  • ch, corresponds to [ʃ] (voiceless postalveolar fricative)
  • ou, corresponds to [u] (close back rounded vowel) or [w] (labio-velar approximant)
  • gn, corresponds to [ɲ] (palatal nasal)
  • qu, corresponds to [k] (voiceless velar stop), typically before historic front vowels

See also French phonology and orthography

• Portuguese
  • ch, like sh in English
  • lh, similar to ll in Spanish, like lli in English million
  • nh, similar to &ntilde in Spanish, like ny in English canyon
  • qu, as k in English
  • sc
  • xc
  • ss, provides for silibant s between two vowels, where single s is pronounced like English z
  • rr, throaty r sound in middle of words

• Spanish
  • ch, corresponds to [tʃ] (voiceless postalveolar affricate)

• Welsh
  • ch, corresponds to [χ] (voiceless uvular fricative), similar to French "r"
  • ng, corresponds to [ŋ] (velar nasal), the same sound as in English
  • ph, corresponds to [f] (voiceless labiodental fricative)
  • rh, corresponds to [r̥] a voiceless R, pronounced roughly like the English combination HR
  • th, corresponds to [θ] (voiceless interdental fricative)

Reversals

Reversals are sequences in which both possible orders of letters are common enough to be digraphs.

• English
  • re corresponds to [rə]
  • le corresponds to [əl]

Doubled Letters

These have both letters the same. In some languages these indicate length, a stressed syllable or a new sound, and in some cases they are just part of the spelling convention. Ll is the most common in English, though it represents no new sound, but that is not the case in other languages; Welsh's ll is a voiceless lateral, and in Spanish it is a palatalized l [ʎ] (Castilian only) or else a palatal fricative. Ee and oo are common examples from English. Rr in Spanish indicates a trill, and forms minimal pairs with the single r. Italian's zz represents the affricate [ʦ].

• English
  • ll corresponds to [l] (voiced alveolar lateral approximant)
  • ee corresponds to [i] (close front unrounded vowel)
  • oo corresponds to [u] (close back unrounded vowel)

• Welsh
  • dd, a voiced dental fricative, like English then
  • ff, the voiceless labiodental fricative, [f] (like English f, as Welsh F is pronounced like English V)
  • ll, a voiceless alveolar lateral fricative, [ɬ] (see Welsh pronunciation guide for more details)

See also

• orthography
• trigraph
• diphthong
• ligature (typography)