Closing, Opening, and Centering
In closing diphthongs, the second element is more close than the first (e.g. ); in opening diphthongs, the second element is more open (e.g. ). Closing diphthongs tend to be falling, and opening diphthongs are generally rising, 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. 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, and in Received Pronunciation or and in Irish. Many centering diphthongs are also opening diphthongs (, ).
diphthongs may contrast in how far they open or close. For example, Samoan contrasts low-to-mid with low-to-high diphthongs:
- ’ai 'probably'
- ’ae 'but'
- ’auro 'gold'
- ao 'a cloud'