A digraph (from Ancient Greek δίς (dís) 'double' and γράφω (gráphō) 'to write') or digram is a pair of characters used in the orthography of a language to write either a single phoneme (distinct sound), or a sequence of phonemes that does not correspond to the normal values of the two characters combined.

Some digraphs represent phonemes that cannot be represented with a single character in the writing system of a language, like ⟨ch⟩ in Spanish chico and ocho. Other digraphs represent phonemes that can also be represented by single characters. A digraph that shares its pronunciation with a single character may be a relic from an earlier period of the language when the digraph had a different pronunciation, or may represent a distinction that is made only in certain dialects, like the English ⟨wh⟩. Some such digraphs are used for purely etymological reasons, like ⟨ph⟩ in French.

In some orthographies, a digraph (or a trigraph) is considered to constitute a letter, which means that it has its own place in the alphabet and cannot be separated into its constituent graphemes for purposes of sorting, abbreviating, or hyphenating words. Digraphs are used in some romanization schemes, e.g. ⟨zh⟩ as a romanisation of Russian ⟨ж⟩.

The capitalisation of digraphs can vary, e.g. ⟨sz⟩ in Polish is capitalized ⟨Sz⟩ and ⟨kj⟩ in Norwegian is capitalized ⟨Kj⟩, while ⟨ij⟩ in Dutch is capitalized ⟨IJ⟩ and word initial ⟨dt⟩ in Irish is capitalized ⟨dT⟩.

Digraphs may also develop into ligatures, but the two concepts are distinct; a digraph's essential feature is its sound, while a ligature is visual, graphically fusing two characters into one, e.g. when ⟨o⟩ and ⟨e⟩ become ⟨œ⟩, e.g. as in French cœur "heart".

Digraphs may consist of two different characters (heterogeneous digraphs) or two instances of the same character (homogeneous digraphs). In the latter case, they are generally called double (or doubled) letters.

Doubled vowel letters are commonly used to indicate a long vowel sound. This is the case in Finnish and Estonian, for instance, where ⟨uu⟩ represents a longer version of the vowel denoted by ⟨u⟩, ⟨ää⟩ represents a longer version of the vowel denoted by ⟨ä⟩, and so on. In Middle English, the sequences ⟨ee⟩ and ⟨oo⟩ were used in a similar way, to represent lengthened "e" and "o" sounds respectively; both spellings have been retained in modern English orthography, but the Great Vowel Shift and other historical sound changes mean that the modern pronunciations are quite different from the original ones.

Doubled consonant letters can also be used to indicate a long or geminated consonant sound. In Italian, for example, consonants written double are pronounced longer than single ones. This was the original use of doubled consonant letters in Old English, but during the Middle English and Early Modern English period, phonemic consonant length was lost and a spelling convention developed in which a doubled consonant serves to indicate that a preceding vowel is to be pronounced short. In modern English, for example, the ⟨pp⟩ of tapping differentiates the first vowel sound from that of taping. In rare cases, doubled consonant letters represent a true geminate consonant in modern English; this may occur when two instances of the same consonant come from different morphemes, for example ⟨nn⟩ in unnatural (un+natural) or ⟨tt⟩ in cattail (cat+tail).