Vowel teams that often make a long vowel sound are also vowel digraphs, so it is helpful to learn these separately. The letters OO can make two different sounds: a short oo like in “foot” or a sound like a long u in “boot.”ĭigraphs are two letters that make one sound, and diphthongs may be called vowel digraphs. The letters AU and AW make the same sound as a short o. These vowel combinations make sounds with mouth positions that change, the definition of a diphthong. OU and OW do too sometimes, although they can make other sounds. The letters OI and OY make the same sound. These vowel combinations are not long or short (mostly), but make completely different vowel sounds. When two letters work together to make one vowel sound that is new, they may be called Diphthongs.