multigraph
sequence of letters that behaves as a unit, not as a sequence of parts
digraph
pair of characters used to write one phoneme
pentagraph
sequence of five letters used to represent a single sound (phoneme), or a combination of sounds, that do not correspond to the individual values of the letters
trigraph
group of three characters used to represent a single sound or a combination of sounds that does not correspond to the written letters combined
digraphs and trigraphs
in computer programming