Basic Grammar: What Is a Diphthong?
by Richard Nordquist/Thought.co
The word “diphthong” comes from the Greek and means “two voices” or “two sounds.” In phonetics, a diphthong is a vowel in which there is a noticeable sound change within the same syllable. (A single or simple vowel is known as a monophthong.) The process of moving from one vowel sound to another is called gliding, which is why another name for a diphthong is a gliding vowel but they are also known as compound vowels, complex vowels, or moving vowels. The sound change that turns a single vowel into a diphthong is called diphthongization. Diphthongs are sometimes referred to as “long vowels” but this is misleading. While vowel sounds do change in a diphthong, they do not necessarily take more time to say than a monophthong.
How many diphthongs are there in the English language? It depends on which expert you ask. Some sources cite eight, others as many as 10. Even syllables containing a single vowel can contain a diphthong. The rule of thumb is: If the sound moves, it’s a diphthong; if it’s static, it’s a monophthong. Each of the following diphthongs is represented by its phonetic symbol.