Vowel hiatus

In phonology, hiatus ( hy-AY-təs) or diaeresis ( dy-ERR-ə-siss, -⁠EER-; also spelled dieresis or diæresis) describes the occurrence of two separate vowel sounds in adjacent syllables with no intervening consonant. When two vowel sounds instead occur together as part of a single syllable, the result is called a diphthong.

Preference

Some languages do not have diphthongs, except sometimes in rapid speech, or they have a limited number of diphthongs but also numerous vowel sequences that cannot form diphthongs and so appear in hiatus. That is the case for Nuosu, Bantu languages like Swahili, and Lakota. An example is Swahili eua 'purify' with three syllables.

Avoidance

Many languages disallow or restrict hiatus and avoid it by deleting or assimilating the vowel sound or by adding an extra consonant sound.

Epenthesis

A consonant sound may be added between vowels (epenthesis) to prevent hiatus. That is most often a semivowel or a glottal, but all kinds of other consonants can be used as well, depending on the language and the quality of the two adjacent vowels. Epenthesis can be seen for instance with intrusive R in multiple varieties of English, in which an R sound is inserted where there is no written R, in order to avoid vowel hiatus. Additionally, some non-rhotic dialects of English often insert a rhotic (/r/) to avoid hiatus after non-high word-final or occasionally morpheme-final vowels, in a process known as linking R.

Contraction

In Greek and Latin poetry, hiatus is generally avoided although it occurs in many authors under certain rules, with varying degrees of poetic licence.