Tetragraphs

A tetragraph, , is a sequence of four letters used to represent a single sound (phoneme), or a combination of sounds, that do not necessarily correspond to the individual values of the letters. In German, for example, the tetragraph tsch represents the sound of the English digraph ch. English does not have tetragraphs in native words (the closest is perhaps the sequence -ough in words like through), but chth and phth are true tetragraphs when found initially in words of Greek origin such as chthonic and phthisis.