The letters A, E, I, O, and U are collectively known as vowels. In English, vowel letters represent sounds made with a relatively unobstructed airflow through the mouth, unlike consonants, which generally involve narrowing or blocking that airflow.
The five-letter list is a traditional guide, not a complete map of English pronunciation. The letter Y can function as a vowel in words such as “myth” and “gym,” while some vowel sounds are written with combinations such as “ai” in “rain” or “ou” in “house.” A single vowel letter can also represent different sounds: the A in “cat,” “cake,” and “father” is pronounced differently.
The word “vowel” comes through Old French and Latin from a term meaning “sounding” or “speaking.” Vowels are central to syllables: every standard English syllable contains a vowel sound, though that sound may be represented by one letter, several letters, or occasionally a syllabic consonant. Diphthongs are not alternatives to vowels; they are moving vowel sounds formed by combining two vowel qualities within one syllable.