Semi-vowels (glide):

It has long been recognised that most languages contain a class of sound that functions in a way similar to consonants but is phonetically similar to vowels: in English, for example, the sounds / w / and / j / (as found in 'wet' and 'yet') are of this type: they are used in the first part of syllables, preceding vowels, but if / w / and / j / are pronounced slowly, it can be clearly heard that in quality they resemble the vowels [u ] and [ i ] respectively. However, a glide is a sound which doesn’t have a steady place of articulation, that is to say, the actual point which the essential vocalic glide begins depends on the nature fo the following sound. The term semi-vowel has been in use for a long time for such sounds, though it is not a very helpful or meaningful name; the term approximant is more often used today.

Despite the fact that semi-vowels or glide are, in phonetic terms, generally vocalic, they are trated within the consonant class, mainly because their function is consonantal rather than vowel-like. Their consonant function is emphasized by the fact that the articles have their preconsonantal form, / ðə / instead of / ði / and the indefinite article "a" rather than "an", when are followed by /j/ and /w/:


Pre-consonantal form:

A year
/ə ˈjɪə || ə ˈjɪər/ --->correct
/ən ˈjɪə || ən ˈjɪər/--->incorrect


The yard
/ ðə jɑːd/ || / ðə jɑːrd/--->correct
/ ði jɑːd/ || / ði jɑːrd/--->incorrect


A wound
/ə ˈwuːnd/--->correct
/ ən ˈwuːnd/--->incorrect

/j/

It is articulated by the tongue assuming the position for the front half-close to close vowel (depending on the degree of opennes of the following sound) and moving away immediately to the position of the following sound.

/w/

It is articulated by the tongue assuming the position for a back half close vowel (depending upon the degree of openness of the following sound) and moving away immediately to the position of the following sound