A digraph, bigraph, or digram (from the , "dís", "double" and γράφω, "gráphō", "write") is a string of two letters where the number of written characters does not match the number of sounds that are spoken. In other words, two graphemes (units of writing, or letters) are used to represent one phoneme (distinct sound). For example, since there are no single letters in English alphabet to represent the sounds /ʃ/, /tʃ/, and /θ/, English uses a string of two letters to represent the sounds in the same order "sh", "ch", and "th." While other sounds such as /f/ and /w/ already have letters that correspond to the sounds, "f" and "w", digraphs "ph" and "wh" are used in some words to show their etymologies (origins of words) rather than their current pronunciation.