A speech sound that is formed with only the most minimal restriction of air is called a vowel. It starts with a restriction of air by the vocal chords, and it passes through the the mouth cavity without restriction. Greek has seven vowels. These are subdivided mainly by length (measured in time air escapes in order to make the sound). The well known English vowels (a e i o u) are shared by Greek, along with two special long vowels--sounds which English does not represent by a separate letter (eta, the long 'e' sound and omega, the long 'o' sound). Sometimes two vowels are pronounced together. These are called diphthongs.