A diphthong is a single vowel sound that begins with one vowel quality and glides toward another within the same syllable. In English, diphthongs do not split into two syllables. They are different from pure vowels, which have a steady quality. Recognizing the glide is key to identifying diphthongs.

Rule
A diphthong is one vowel sound that moves from one quality to another within a syllable.
In English, diphthongs stay inside one syllable and do not become two separate vowels.
Pure vowels have a steady sound, but diphthongs have a clear glide.

English has several common diphthongs that appear in many accents. The set below covers the core diphthongs found in General American and Received Pronunciation. Each diphthong is named by its starting and ending vowel qualities. Accurate pronunciation depends on making both parts of the glide clear.

Word/Phrase	Definition
/aɪ/	The vowel sound in words like price and my glides from an open front to a close front quality.
/eɪ/	The vowel sound in words like face and day glides from a mid front to a close front quality.
/ɔɪ/	The vowel sound in words like choice and boy glides from a mid or open-mid back to a close front quality.
/aʊ/	The vowel sound in words like mouth and now glides from an open front or central to a near-close back quality.
/əʊ/	The vowel sound in words like goat and go glides from a mid central to a close-mid back quality.
/oʊ/	The vowel sound in words like goat and go glides from a mid back to a near-close back quality.
/ɪə/	The vowel sound in words like near and here glides from a close front to a mid or central quality.
/eə/	The vowel sound in words like square and air glides from a mid front to a mid central quality.
/ʊə/	The vowel sound in words like cure and tourist glides from a near-close back to a mid or central quality.

Diphthongs are often confused with pure vowels that sound similar at the start or end of the glide. The difference is that a diphthong moves clearly within the syllable, while a pure vowel stays stable. In English, meaning can change if the glide is missing or replaced.

Rule
A diphthong requires a movement between two vowel qualities; a pure vowel does not move.
In English, mispronouncing a diphthong as a pure vowel can make words hard to recognize.
The beginning and ending targets of the glide both need to be present for a clear diphthong.

To pronounce a diphthong, shape the mouth for the starting vowel and then move smoothly toward the ending vowel within the same syllable. The movement can be more or less pronounced depending on the diphthong. In English, the glide is often quicker than saying two separate vowels. Maintaining one syllable timing helps keep the diphthong distinct from a vowel sequence.

Rule
Start with the first vowel target, then move toward the second target without a break.
Keep the entire diphthong inside one syllable length.
Avoid inserting a pause or extra sound between the two parts of the glide.

Some English diphthongs change their exact quality or length depending on the accent. For example, the vowel in goat is often /əʊ/ in Received Pronunciation and /oʊ/ in General American. Some diphthongs like /ɪə/, /eə/, and /ʊə/ are more prominent in British varieties and may be simplified in American speech.

Region	Word/Phrase	Regional Definition
🇬🇧Received Pronunciation	/əʊ/	This is the typical diphthong for goat words in RP, gliding from mid central to close-mid back.
🇺🇸General American	/oʊ/	This is the typical diphthong for goat words in GA, gliding from mid back to near-close back.
🇺🇸General American	/ɪə/	This diphthong is often replaced by a sequence or a tense vowel in GA, especially in words like near.
🇺🇸General American	/eə/	This diphthong is often replaced by /ɛr/ in GA, as in air and square.
🇺🇸General American	/ʊə/	This diphthong is often merged or replaced in GA, so cure may sound closer to /ʊr/.

English diphthongs are single syllable vowel sounds that glide between two qualities. Mastery comes from hearing the movement and producing a smooth, quick glide. Distinguishing diphthongs from pure vowels supports clearer pronunciation and listening. Accent differences affect which diphthongs are most common, but the principle of the glide stays the same.