pairs of letters which can occur in the plaintext language (ie English). See here for common digraphs

Two successive letters that make a single sound (e.g., ea in bread or ng in sing).

are graphemes written with more than one letter. Digraphs can represent consonants (ch, sh, th, ck, ng, dge, etc.) or vowels (ai, ay, ee, ea, igh, ew, etc.).

Two consecutive consonants representing one sound. (sh, ch, th, gh, ph, wh)

A plaintext character pairing technique that prevents frequency analysis of commonly occurring pairs such as 'qu'. Note that trigraphs (three characters at a time) is an extension of the theme.

Two letters that represent one speech sound, such as ch for /ch/ in chin or ea for in bread.