When a sound consists of a single vowel it is called a pure vowel sound, of which there are 12. A diphthong is a combination of two vowel sounds produced as a single syllable, where one vowel sound transitions to another. Diphthongs are also called impure vowel sounds. There are 9 diphthongs that are combinations of vowel sounds like "ei" in "day" and "ou" in "hold". While there are 21 consonant letters, there are 27 consonant sounds due to combinations like "dw" and "kw" that represent single consonant sounds.