Pentagraph

A pentagraph (from the Greek:πέντε, pénte, "five" and γράφω, gráphō, "write") is a 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. In German, for example, the pentagraph tzsch represents the [tʃ] sound of the English digraph ch, and indeed is found in the English word Nietzschean. Irish has a number of pentagraphs. English, like most languages, has none.