Glides, also known as semi-vowels, are neither vowels nor consonants. Though they share the feature -consonantal, they sometimes pattern more like consonants than like vowels. For example, they are -syllabic. For this reason, phonologists are often not entirely sure how to classify them.
In French, the indefinite article "un" is pronounced with a nasal before a noun that starts with a vowel. For example, "un enfant" ( a child) has a nasal segment whereas "un lit" (a bed) only has a fully nasalized vowel. The phrase "un oiseau" (a bird), however, is pronounced with a nasal before the labiovelar glide of "oiseau". Here the glide patterns like a vowel. In the word "wow", the first glide is more like a consonant. It is syllable-peripheral and easily perceived. However, the word-final glide behaves more like a vowel. It is the second component of a diphthong and is not easily perceived as separate from the first component.
With respect to acoustic turbulence or obstruction through the oral cavity, glides are similar to vowels. Plosives have the most obstruction followed by affricates, fricatives, nasals, liquids, glides and vowels. In this respect, glides are very much like vowels.
However, other examples show they can behave like consonants. For example, in Danish the "d" is usually a fricative between vowels as in "bide" (to bite). However, the word "arbejde" (to work) has an alveolar plosive, not a fricative. This is also the case in "burde" (ought to), in which the "d" is a voiced plosive.
In Argentinian Spanish, nasals assimilate to the place of articulation of a glide but not a vowel. For example, "con hielo" (with ice), has a palatal nasal followed by a palatal glide, but "con anteojos" (with glasses) is simply a dental nasal. This analysis may be complicated by the fact that the palatal glide of Spanish is often articulated more like an approximant or fricative, however. Nevertheless, this nasal place assimilation is typically associated with consonants.
In Danish the word "liv" (life) is often vocalized so that the "v" forms a diphthong with the "i", but the word "livet" consists of a labiodental fricative. The vocalization occurs syllable-finally, a position where the glide is difficult to perceive separately from the vowel, but also like a consonant in the sense that it is syllable peripheral. In the word "livet", the "v" is syllable-initial and does not vocalize. Thus, it appears that glides tend to behave more like consonants in syllable-initial position than in syllable-final.
In Japanese, the palatal glide has a relatively long duration and in careful speech does not diphthongize. This is exemplified by the word "hai" meaning yes. Also, the glide has a mora of its own. The mora can be described as a unit which is between a segment and a syllable. In the sense that the glide has its own mora, it behaves as a consonant.
Glides, also known as semi-vowels, are fascinating sounds which are relatively difficult to classify. Though they are -consonantal, thus similar to vowels, they are also -syllabic which makes them similar to consonants. They tend to immediately precede or follow vowels, a characteristic which makes them similar to vowels. However, in many ways, they behave like consonants. They occur syllable-peripherally, they have their own mora in Japanese and block spirantization of the Danish "d" when it occurs between a glide and a vowel. Thus, they often pattern like consonants.