Digraph

A digraph is a pair of letters used to write one sound. This is often a sound or more precisely a phoneme which cannot be expressed using a single letter in the Latan alphabet.

English is an oddity in that there are, for historical reasons, often many ways to write the same sound. It is not unique in this regard--in Spanish, s and z represent the same sound--but there are more such sounds in English than in most languages that use the Latin alphabet.

See also: orthography