A diphthong is a complex vowel sound created by blending two distinct vowel timbres within one syllable, such as the "oi" in "coin" or "ou" in "house." In poetry, diphthongs affect syllable counting and metrical scansion because they function as single units despite their dual vowel composition; understanding diphthongs is crucial for accurate meter analysis, especially in verse that relies on strict syllable counts like iambic pentameter.

A diphthong is a complex vowel sound formed by the combination of two adjacent vowel sounds within the same syllable (e.g., "oi" in "coin" or "ou" in "out"). In prosody, diphthongs are treated as single syllabic units despite their internal glide, which affects metrical scansion—particularly in quantitative meters like those of Greek and Latin poetry, where syllable length matters. English prosody, being stress-based, treats diphthongs as one stressed or unstressed syllable depending on context. Misidentifying diphthongs as two separate vowels can lead to incorrect scansion or forced elision. Related terms include hiatus (two adjacent vowels in separate syllables) and synizesis (the compression of two vowels into one syllable).