A diphthong is a sound produced when two vowels are pronounced together, creating a new sound for a language. The word is a compound word, from the Greek words for "two" and "voice."

There are eight such combinations of vowels in Greek, as listed below, all of which have English equivalents. The proper pronunciation of some of the diphthongs is disputed, particularly for ai, au and ou.

1. Diphthongs end in u or i.