diphthong (n.)

A term used in the PHONETIC classification of VOWEL sounds on the basis of their MANNER OF ARTICULATION: it refers to a vowel where there is a single (perceptual) noticeable change in quality during a SYLLABLE, as in English beer, time, loud. Related terms are MONOPHTHONG, where no qualitative change is heard, and TRIPHTHONG, where two such changes can be heard. Diphthongs, or ‘gliding vowels’, are usually classified into phonetic types, depending on which of the two elements is the more SONOROUS: ‘falling’ (or ‘descending’) diphthongs have the first element STRESSED, as in the English examples: ‘rising’ (or ‘ascending’) diphthongs have the second element stressed, as in a possible analysis of English cue [kiu].

Other classifications of diphthongal types exist, in terms of the extent of their movement (e.g. whether it is ‘wide’ or ‘narrow’) and their direction (whether the diphthong is ‘centring’ or not, i.e. ending with a CENTRAL vowel). Diphthongization is the term used to describe a process where a monophthong has become a diphthong (has been diphthongized), as in cases of historical or DIALECT change. Diphthongs are transcribed using symbols which represent the extremes of vowel movement between the two positions, as in [aI] for the unit in fine.