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In phonetics, a diphthong, ˈdɪf.θɒŋ or /ˈdɪp.θɒŋ/, (also gliding vowel) (from Greek grc δίφθογγος, ''diphthongos'', literally ''two sounds'' or ''two tones'') is a contour vowel—that is, a unitary vowel that changes quality during its pronunciation, or ''glides'', with a smooth movement of the tongue from one articulation to another, as in the English words ''eye, boy,'' and ''cow.'' This contrasts with ''pure'' vowels, or ''monophthongs,'' where the tongue is held still, as in the English word ''papa.''Diphthongs often form when separate vowels are run together in rapid speech. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single vowel sounds (phonemes).

In the International Phonetic Alphabet, pure vowels are transcribed with one letter, as in English ''sum'' [sʌm]. Diphthongs are transcribed with two letters, as in English ''eye'' [aɪ̯] or ''same'' [seɪ̯m]. The two vowel symbols are chosen to represent the beginning

and ending positions of the tongue, though this can be only approximate. The diacritic is placed under the less prominent component to show that it is part of a diphthong rather than a separate vowel, though it is sometimes left off in languages such as English, where there is not likely to be any confusion. (That is, in precise transcription, [ai] represents two vowels in hiatus, as found for example in Hawaiian and in the English word ''naïve''. It would not be a diphthong, for instance, in the English word, ''knives'').

Types of diphthong

Falling (or descending) diphthongs start with a vowel quality of higher prominence (higher pitch or louder) and end in a semivowel with less prominence, like [aɪ̯] in ''eye'', while rising (or ascending) diphthongs begin with a less prominent semivowel and end with a more prominent full vowel, like [ɪ̯a] in ''yard''. The less prominent component in the diphthong may also be transcribed as an approximant, thus [aj] in ''eye'' and [ja] in ''yard''. However, when the diphthong is analysed as a single phoneme, both elements are often transcribed with vowel letters . Note also that semivowels and approximants are not equivalent in all treatments, and in the English and Italian languages, among others, many phoneticians do not consider rising combinations to be diphthongs, but rather sequences of approximant and vowel. There are many languages (such as Romanian) that contrast one or more rising diphthongs with similar sequences of a glide and a vowel in their phonetic inventory.

In closing diphthongs, the second element is more close than the first (e.g. [ai]); in opening diphthongs, more open (e.g. [ia]). Closing diphthongs tend to be falling , and opening diphthongs are generally rising , because open vowels are more sonorous and therefore tend to be more prominent. However, exceptions to this rule are not rare in the world's languages. In Finnish, for instance, the opening diphthongs /ie̯/ and /uo̯/ are true falling diphthongs, since they begin louder and with higher pitch and fall in prominence during the diphthong.

A centering diphthong is one that begins with a more peripheral vowel and ends with a more central one, such as [ɪə̯], [ɛə̯], and [ʊə̯] in Received Pronunciation or [iə̯] and [uə̯] in Irish. Many centering diphthongs are also opening diphthongs .

Some languages contrast short and long diphthongs, the latter usually being described as having a long first element. Languages that contrast three quantities in diphthongs are extremely rare, but not unheard of: Northern Sami is known to contrast long, short and finally stressed diphthongs, the last of which are distinguished by a long second element.

While there are a number of similarities, diphthongs are not the same as a combination of a vowel and a semivowel or glide. Most importantly, diphthongs are contained in the syllable nucleus while a semivowel or glide is restricted to the syllable boundaries (either the onset or the coda). This often manifests itself phonetically by a greater degree of constriction. though this phonetic distinction is not always clear. The English word ''yes'', for example, consists of a palatal glide followed by a monophthong rather than a rising diphthong.

Diphthongs in various languages

Catalan

Catalan possesses a number of phonetic diphthongs, all of which begin or end in [j] or [w]. They include:

In addition to these, Catalan also possesses two sets of diphthongs in variation; [wi] varies with [uj] (as in ''afluixar'' [aflujˈɕa~aflwiˈɕa] 'to loosen') and [iw] with [ju].

There are also certain instances of ''compensatory diphthongization'' in the Majorcan dialect so that /ˈtroncs/ ('logs') (in addition to deleting the palatal plosive) develops a compensating palatal glide and surfaces as [ˈtrojns] (and contrasts with the unpluralized [ˈtronʲc]). Diphthongization compensates for the loss of the palatal stop (part of Catalan's segment loss compensation). There are other cases where diphthongization compensates for the loss of point of articulation features (property loss compensation) as in [ˈaɲ] ('year') vs [ˈajns] ('years').

The dialectal distribution of compensatory diphthongization is almost entirely dependent on the dorsal plosive (whether it is velar or palatal) and the extent of consonant assimilation (whether or not it's extended to palatals).

Croatian

Croatian dialects also have ''uo'', as in ''kuonj, ruod, uon'' while, in Standard Croatian, these words are konj, rod, on)

Czech

There are three diphthongs in Czech:

Vowel groups ''ia, ie, ii, io'', and ''iu'' in foreign words are not regarded as diphthongs, they are pronounced with /j/ between the vowels [ɪja, ɪjɛ, ɪjɪ, ɪjo, ɪju].

Dutch

The dialect of Hamont (in Limburg) has five centring diphthongs and contrasts long and short forms of [ɛɪ̯], [œʏ̯], [ɔʊ̯], and [ɑʊ̯].

English

All English diphthongs are falling, apart from /juː/, which can be analyzed as [i̯uː].

Faroese

Diphthongs in Faroese are:

Finnish

All Finnish diphthongs are falling. Notably, Finnish has true opening diphthongs (e.g. /uo/), which are not very common crosslinguistically compared to centering diphthongs (e.g. /uə/ in English).

French

Some diphthongs in French:

While /wa/, /wɛ̃/, and /ɥi/ may be considered diphthongs (that is, fully contained in the syllable nucleus), other sequences of a glide and vowel are considered part of a glide formation process that turns a high vowel into a glide (and part of the syllable onset) when followed by another vowel.

German

Diphthongs in German:

Some diphthongs in Bernese, a Swiss German dialect: