International Phonetic Alphabet

Diphthong
A diphthong (pronounced /ˈdɪfθɒŋ/ or /ˈdɪpθɒŋ/;[1] from Greek δίφθογγος, diphthongos, literally "two sounds" or "two tones"), also known as a gliding vowel, refers to two adjacent vowel sounds occurring within the same syllable. Technically, a diphthong is a vowel with two different targets: That is, the tongue moves during the pronunciation of the vowel. In most dialects of English, the words eye, hay, boy, low, and cow contain diphthongs.

Diphthongs contrast with monophthongs, where the tongue doesn't move and only one vowel sound is heard in a syllable. Where two adjacent vowel sounds occur in different syllables, as in, for example, the English word re-elect, the result is described as hiatus, not as a diphthong.

Diphthongs often form when separate vowels are run together in rapid speech during a conversation. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single-vowel sounds (phonemes)

International Phonetic Alphabet

In the International Phonetic Alphabet, pure vowels are transcribed with one letter, as in English sun [sʌn]. Diphthongs are transcribed with two letters, as in English sign [saɪ̯n] or sane [seɪ̯n]. The two vowel symbols are chosen to represent the beginning and ending positions of the tongue, though this can be only approximate.

The non-syllabic diacritic (an inverted breve below) ‹◌̯› is placed under the less prominent component to show that it is part of a diphthong rather than a separate vowel. It is, however, sometimes omitted in languages such as English, where there is not likely to be any confusion.

In precise transcription, ‹ai› without this diacritic represents two vowels in hiatus, found for example in Hawaiian and in the English word naïve, and does not represent the diphthong of the Finnish word laiva [lai̯ʋa] "ship".

Types
Falling and rising

Falling (or descending) diphthongs start with a vowel quality of higher prominence (higher pitch or volume) 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, similar to the [ja] in yard. (Note that "falling" and "rising" in this context do not refer to vowel height; the terms "opening" and "closing" are used instead. See below.) 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 (/aɪ̯/, /ɪ̯a/). 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[3] (see semivowel for examples).

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Closing, opening, and centering

In closing diphthongs, the second element is more close than the first (e.g. [ai]); in opening diphthongs, the second element is more open (e.g. [ia]). Closing diphthongs tend to be falling ([ai̯]), and opening diphthongs are generally rising ([i̯a]), as 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 third, rare type of diphthong that is neither opening nor closing is height-harmonic diphthongs, with both elements at the same vowel height.[citation needed] These were particularly characteristic of Old English, which had diphthongs such as /æɑ̯/, /eo̯/.

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 ([iə̯], [uə̯]).

Diphthongs may contrast in how far they open or close. For example, Samoan contrasts low-to-mid with low-to-high diphthongs:

’ai [ʔai̯] 'probably'
’ae [ʔae̯] 'but'
’auro [ʔau̯ɾo] 'gold'
ao [ao̯] 'a cloud'
Length

Languages differ in the length of diphthongs, measured in terms of morae. In languages with phonemically short and long vowels, diphthongs typically behave like long vowels, and are pronounced with a similar length.[citation needed] In languages with only one phonemic length for pure vowels, however, diphthongs may be behave like pure vowels.[citation needed] For example, in Icelandic, both monophthongs and diphthongs are pronounced long before single consonants and short before most consonant clusters.

Some languages contrast short and long diphthongs. In some languages, such as Old English, these behave like short and long vowels, occupying one and two morae, respectively. In other languages, however, such as Ancient Greek, they occupy two and three morae, respectively, with the first element rather than the diphthong as a whole behaving as a short or long vowel.[citation needed] 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.[citation needed]

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Difference from a vowel and semivowel

While there are a number of similarities, diphthongs are not the same as a combination of a vowel and an approximant or glide. Most importantly, diphthongs are fully contained in the syllable nucleus[4][5] 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.[6] though this phonetic distinction is not always clear.[7] The English word yes, for example, consists of a palatal glide followed by a monophthong rather than a rising diphthong. In addition, while the segmental elements must be different in diphthongs so that [ii̯], when it occurs in a language, does not contrast with [iː] though it is possible to contrast [ij] and [iː].[8]

Nonetheless, in practice the choice of treating a diphthong or diphthong-like element as a single phoneme, a sequence of two vowels or a combination of a vowel and a glide is based not on the phonetic nature of the diphthong but on systemic properties of the language.[citation needed] The following are examples of systemic characteristics that tend to determine which analysis is chosen:[citation needed]

The presence of alternations among related words or related dialects between diphthongs and monophthongs, sequences of vowel and consonant, or sequences of two vowels in separate syllables
The restrictions (or lack thereof) on the diphthongs that can occur
The existence of glides such as /w/ and /j/ as separate phonemes in the language
The behavior of the diphthong when a vowel directly follows
The historical origin of the diphthong

Furthermore, falling diphthongs are more likely to be analyzed as unit phonemes than rising diphthongs.

As an example, the English diphthongs are usually considered single phonemes because they (mostly) originated historically as monophthongs, alternate with monophthongs in pairs such as divine vs. divinity, maintain their coherence when another vowel follows, and other, similar-looking diphthongs like /eu/ do not exist in the language.[citation needed] On the other hand, Japanese /ai/ is normally analyzed as a sequence of two vowels; Spanish /ai/ is normally analyzed as either a sequence of two vowels or of a vowel and a glide, depending on the analysis.