Trigraph (orthography)
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A trigraph (from Ancient Greek τρεῖς (treîs) 'three', and γράφω (gráphō) 'to write')
digraph (from Ancient Greek δίς (dís) 'double', and γράφω (gráphō) 'to write, draw, paint, etc.')) is a group of three characters used to represent a single sound or a combination of sounds that does not correspond to the written letters combined.