Question: what are vowel digraphs and diphthongs?
Answer: Two vowel letters that spell a single sound are called a digraph. Vowel digraphs include oi and oy /oi/ as in boil and boy, and ow and ou /ou/ as in cow and out. (The sounds /oi/ and /ou/ are commonly called diphthongs. A diphthong is a speech sound in which a first vowel sound glides into a second.)