Diftongs

Diftongs, more commonly known as diphthongs, are vowel sounds that begin with one vowel and glide to another within the same syllable. The term comes from the Greek diphthongos, meaning two sounds. Diphthongs contrast with monophthongs, which have a single, unchanging vowel quality, and with triphthongs, which involve a glide through three vowel qualities in one syllable.

In many languages, including English, diphthongs function as phonemes capable of separate meaning. They are often described by their start and end points, or by the direction of the glide. Common English examples include /eɪ/ as in "face," /aɪ/ as in "price," /ɔɪ/ as in "choice," /aʊ/ as in "mouth," and /oʊ/ as in "go." Some dialects also feature centering diphthongs such as /ɪə/ or /eə/ in certain words, though the exact inventory varies by dialect.

Phonologically, diphthongs can be phonemic—distinguishing words in a language—or allophonic variants of a single phoneme. In writing systems, they are often indicated by digraphs or diacritics, but orthography does not always reveal their presence. Diphthongs occur in many language families, with substantial variation in their size, starting points, and trajectories. Some languages have many diphthongs and few pure vowels, while others preferentially use monophthongs. Understanding diphthongs is essential for accurate pronunciation, transcription, and phonological analysis.