Semivowel

In phonetics and phonology, a semivowel, glide or semiconsonant is a sound that is phonetically similar to a vowel sound but functions as the syllable boundary, rather than as the nucleus of a syllable. Examples of semivowels in English are the consonants y and w in yes and west, respectively. Written in IPA, y and w are near to the vowels ee and oo in seen and moon, written  in IPA. The term glide may alternatively refer to any type of transitional sound, not necessarily a semivowel.

Classification
Semivowels form a subclass of approximants. Although "semivowel" and "approximant" are sometimes treated as synonymous, most authors use the term "semivowel" for a more restricted set; there is no universally agreed-upon definition, and the exact details may vary from author to author. For example, do not consider the labiodental approximant  to be a semivowel.

In the International Phonetic Alphabet, the diacritic attached to non-syllabic vowel letters is an inverted breve placed below the symbol representing the vowel:. When there is no room for the tack under a symbol, it may be written above, using. Before 1989, non-syllabicity was represented by, which now stands for extra-shortness.

Additionally, there are dedicated symbols for four semivowels that correspond to the four close cardinal vowel sounds:

In addition, some authors consider the rhotic approximants, to be semivowels corresponding to R-colored vowels such as. An unrounded central semivowel, (or ), equivalent to, is uncommon, though rounded  (or ), equivalent to , is found in Swedish and Norwegian.

Contrast with vowels
Semivowels, by definition, contrast with vowels by being non-syllabic. In addition, they are usually shorter than vowels. In languages such as Amharic, Yoruba, and Zuni, semivowels are produced with a narrower constriction in the vocal tract than their corresponding vowels. Nevertheless, semivowels may be phonemically equivalent with vowels. For example, the English word fly can be considered either as an open syllable ending in a diphthong or as a closed syllable ending in a consonant.

It is unusual for a language to contrast a semivowel and a diphthong containing an equivalent vowel, but Romanian contrasts the diphthong with, a perceptually similar approximant-vowel sequence. The diphthong is analyzed as a single segment, and the approximant-vowel sequence is analyzed as two separate segments.

In addition to phonological justifications for the distinction (such as the diphthong alternating with in singular-plural pairs), there are phonetic differences between the pair:
 * has a greater duration than
 * The transition between the two elements is longer and faster for than  with the former having a higher F2 onset (greater constriction of the articulators).

Although a phonological parallel exists between and, the production and perception of phonetic contrasts between the two is much weaker, likely because of lower lexical load for , which is limited largely to loanwords from French, and speakers' difficulty in maintaining contrasts between two back rounded semivowels in comparison to front ones.

Contrast with fricatives/spirant approximants
According to the standard definitions, semivowels (such as ) contrast with fricatives (such as ) in that fricatives produce turbulence, but semivowels do not. In discussing Spanish, Martínez Celdrán suggests setting up a third category of "spirant approximant", contrasting both with semivowel approximants and with fricatives. Though the spirant approximant is more constricted (having a lower F2 amplitude), longer, and unspecified for rounding (viuda 'widow' vs. ayuda  'help'), the distributional overlap is limited. The spirant approximant can only appear in the syllable onset (including word-initially, where the semivowel never appears). The two overlap in distribution after and : enyesar  ('to plaster') aniego  ('flood') and although there is dialectal and idiolectal variation, speakers may also exhibit other near-minimal pairs like abyecto ('abject') vs. abierto ('opened'). One potential minimal pair (depending on dialect) is ya visto ('already seen') vs. y ha visto  ('and he has seen'). Again, it is not present in all dialects. Other dialects differ in either merging the two or enhancing the contrast by moving the former to another place of articulation, like in Rioplatense Spanish.