What is a digraph? A digraph is a single sound represented by two letters. Some examples of these are: "ch" in chicken, "th" in thumb, "ph" in phone, and "wh" in whale. What is a trigraph? A trigraph is a single sound represented by three letters. Some examples of these are: "str" in string, "sch" in school, "sph" in sphere, and "spl" in splash.