1. Diphthongs

A. Definition - a diphthong  (Greekdiphthongos, literally "two sounds" or "two tones"), also known as a gliding vowel, refers to two adjacent vowel sounds occurring within the same syllable. Diphthong is a sound which consist of a movement or glide from one vowel to another. A vowel which remains constant and does not glide is called a pure vowel, and one of the most common pronunciation mistakes that result in a learner of English having a "foreign" accent is the production of pure vowels where a diphthong should be pronounced.
Technically, a diphthong is a vowel with two different targets: that is, the tongue (and/or other parts of the speech apparatus) moves during the pronunciation of the vowel.
Perhaps the most important thing to remember about all the diphthongs is that the first part is much longer and stronger than the second part

Diphthongs contrast with monophthongs, where the tongue or other speech organs do not move and the syllable contains only a single vowel sound. For instance, in English, the word ah is spoken as a monophthong /ɑː/, while the word ow is spoken as a diphthong /aʊ/. Where two adjacent vowel sounds occur in different syllables—for example, in the English word re-elect—the result is described as hiatus (a pause or gap in a sequence, series, or process), 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).

B. Transcription - In the International Phonetic Alphabet (IPA), monophthongs are transcribed with one symbol, as in English sun [sʌn], in which ʌ represents a monophthong. Diphthongs are transcribed with two symbols, as in English high [haɪ] or cow [kaʊ], in which  and  represent diphthongs.
Diphthongs may be transcribed with two vowel symbols or with a vowel symbol and a semivowel symbol. In the words above, the less prominent member of the diphthong can be represented with the symbols for the palatal approximant [j] and the labiovelar approximant [w], with the symbols for the close vowels [i] and [u], or the symbols for the near-close vowels [ɪ] and [ʊ]:

vowel and semivowel
haj kaw
broader transcription
two vowel symbols
hai̯ kau̯
haɪ̯ kaʊ̯
narrower transcription

Some transcriptions are broader or narrower (less precise or more precise phonetically) than others. Transcribing the English diphthongs in high and cow as aj aw or ai̯ au̯ is a less precise or broader transcription, since these diphthongs usually end in a vowel sound that is opener than the semivowels [j w] or the close vowels [i u]. Transcribing the diphthongs as aɪ̯ aʊ̯ is a more precise or narrower transcription, since the English diphthongs usually end in the near-close vowels [ɪ ʊ].

The non-syllabic diacritic, the inverted breve below ◌̯, is placed under the less prominent part of a diphthong to show that it is part of a diphthong rather than a vowel in a separate syllable: [aɪ̯ aʊ̯]. When there is no contrastive vowel sequence in the language, the diacritic (sign indicating a difference in pronunciation) may be omitted. Other common indications that the two letters are not separate vowels are a superscript (of a letter, figure, or symbol written or printed above the line), aᶦ aᶷ, or a tie bar, a͡ɪ a͡ʊ or a͜ɪ a͜ʊ. The tie bar can be useful when it's not clear which letter represents the syllable nucleus, or when they have equal weight. Superscripts are especially used when an on- or off-glide is particularly fleeting.
The period . is the opposite of the non-syllabic diacritic: it represents a syllable break. If two vowels next to each other belong to two different syllables (hiatus), meaning that they do not form a diphthong, they can be transcribed with two vowel symbols with a period in between. Thus, lower can be transcribed ˈloʊ.ər, with a period separating the first syllable, /loʊ/, from the second syllable, /ər/.

C. Types of diphthongs

Falling and rising diphthongs

- 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

- rising diphthongs - 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; for that, 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 (see semivowel for examples).

Closing, opening, and centering diphthongs

- 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.
A third, rare type of diphthong that is neither opening nor closing is height-harmonic diphthongs, with both elements at the same vowel height. These occurred in Old English:
beon [beo̯n] "be"
ceald [kæɑ̯ld] "cold"

- 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]).

Narrow and wide diphthongs

- narrow diphthongs - are the ones that end with a vowel which on a vowel chart is quite close to the one that begins the diphthong, for example Northern Dutch [eɪ], [øʏ] and [oʊ]. 

- wide diphthongs - are the opposite - they require a greater tongue movement, and their offsets are farther away from their starting points on the vowel chart. Examples of wide diphthongs are RP/GA English [aɪ] and [aʊ].

1. [ia] (example words: beard, Ian, fierce) - a centring diphthong glide towards the a (schwa). The starting point is a little closer than i.

2. [ea] (aired, cairn, scarce)a centring diphthong begins with the same vowel sound as the e. 

3. [ua] (moored, tour)this centring diphthong has a starting point slightly closer than u.

4. [ei] (paid, pain, face) - a closing diphthong (they all have the characteristic that they all end with a glide towards a closer vowel). The starting point is the same as the e.

5. [ai] (tide, time, nice) - this closing diphthong begins with an open vowel which is between front and back. It is quite similar to the  Ʌ.

6. [ɔi] (void, loin, voice) - this closing diphthong has the same quality as ɔ.

7. [au] (load, home, most) - the vowel position for the beginning of this is the same as for the "schwa" vowel, as found in the first syllable of the word "about. The lips may be slightly rounded in anticipation of the glide towards u, for which there is quite noticeable lip-rounding.

8. [au] (loud, gown, house) - this diphthong begins with a vowel similar to a: but a little more front. Since this is an open vowel, a glide to u would necessitate a large movement. Usually in English the glide towards u begins but is not completed, the end of the diphthong being somewhere between close -mid and open-mid in tongue height. There is only slight lip-rounding.     

D. 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. In languages with only one phonemic length for pure vowels, however, diphthongs may behave like pure vowels. 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. 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.

Phonology aspect - in some languages, diphthongs are single phonemes, while in others they are analyzed as sequences of two vowels, or of a vowel and a semivowel.

E. Sound changes - certain sound changes relate to diphthongs and monophthongsVowel breaking or diphthongization is a vowel shift in which a monophthong becomes a diphthong. Monophthongization or smoothing is a vowel shift in which a diphthong becomes a monophthong.

Difference from a vowel and semivowel - while there are a number of similarities, diphthongs are not the same phonologically as a combination of a vowel and an approximant or glide. Most importantly, diphthongs are fully 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 the 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. In addition, 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 for languages to contrast [ij] and [iː].

Examples in English - all English diphthongs are falling, apart from /juː/, which can be analyzed as [i̯uː].
In words coming from Middle English, most cases of the Modern English diphthongs [aɪ̯, oʊ̯, eɪ̯, aʊ̯] originate from the Middle English long monophthongs [iː, ɔː, aː, uː] through the Great Vowel Shift, although some cases of [oʊ̯, eɪ̯] originate from the Middle English diphthongs [ɔu̯, aɪ̯].

Standard English diphthongs
North American













2. Triphthongs

In phonetics, a triphthong  (from Greek τρίφθογγος, "triphthongos", literally "with three sounds," or "with three tones") is a monosyllabic vowel combination involving a quick but smooth movement of the articulator from one vowel quality to another that passes over a third. While "pure" vowels, or monophthongs, are said to have one target articulator position, diphthongs have two, and triphthongs three. They are the most complex English sounds of the vowel type. They can be rather difficult to pronounce, and very difficult to recognise. 

The principal cause of difficulty for the foreign learner is that in present-day English the extent of the vowel movements is very small, except in very careful pronunciation. Because of this, the middle of the three vowel qualities of the triphthong can hardly be heard and the resulting sound is difficult to distinguish from some of the diphthongs and long vowels.

Examples in English - in British Received Pronunciation, (monosyllabic triphthongs with R are optionally distinguished from sequences with disyllabic realizations)
[aʊ̯ə̯] as in hour (compare with disyllabic "shower" [aʊ̯.ə])
[aɪ̯ə̯] as in fire (compare with disyllabic "higher" [aɪ̯.ə])
[ɔɪ̯ə̯] as in "loir" (compare with final disyllabic sequence in "employer" [ɔɪ̯.ə])
As [eɪ̯] and [əʊ̯] become [ɛə̯] and [ɔː] respectively before /r/, all instances of [eɪ̯.ə] and [əʊ̯.ə] are words with the suffix "-er".

In Cockney, triphthongal realizations [ɪi̯ɐ̯, ɛi̯ə̯, ɔu̯ə̯, æi̯ə̯] of /iə, eə, ɔə, æʊ/ are possible, and are regarded as "very strongly Cockney". Among these, the triphthongal realization of /ɔə/ occurs most commonly. There is not a complete agreement about the distribution of these; according to Wells (1982b), they "occur in sentence-final position”, whereas according to Mott (2012), these are "most common in final position".

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