Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.
- Three letters spelling one consonant, vowel, or diphthong, such as Sch in Schiller or igh in high or thigh.
- A group of three letters, especially of frequent occurrence in a given language, as the or ing in English or gli in Italian.
- Any combination of three letters of an alphabet. Also called trigram.