A diphthong is a special type of vowel sound that starts as one vowel and glides smoothly into another, all within a single syllable. Your mouth actually changes position as you make the sound, creating a smooth transition between two vowel sounds.

You use diphthongs constantly without realizing it. When you say “my,” your mouth starts with an “ah” sound and glides up to an “ee”-like sound. Try saying it slowly and you'll feel your tongue and lips moving. The same kind of glide happens in words like “cow” (which moves from an “ah”-like sound toward an “oo”-like sound) and “boy” (which moves from an “aw”-like sound toward an “ee”-like sound).

Diphthongs are different from two separate vowel sounds bumping into each other across syllables, like in the word “doing” (do-ing) or “create” (cre-ate). In those words, you're saying two distinct syllables. But in a diphthong, both vowel sounds happen together in one flowing syllable.

English has quite a few diphthongs, which is part of what makes English spelling so tricky. Different letter combinations can create the same diphthong sound, like the “oy” in “boy” and the “oi” in “coin.” Understanding diphthongs helps you notice the interesting patterns in how words actually sound versus how they're spelled.