Letter units that may contain one or more letters, but that map to a single sound (e.g., k, ch). They not only include letters, but also Chinese ideograms, numerals, punctuation marks, and other symbols. A digraph contains two graphemes for one phoneme, and a triagraph three graphemes for one phoneme (e.g., itch in witch).
See Decoding ability, Diagraph, Grapheme-phoneme correspondences, Orthographic reading skills, Orthography, Phoneme