Glides, /w/ and /j/ 

/w/ and /j/ are called glides or semivowels. The letter W is often pronounced as /w/, and letter Y as /j/. /w/ and /j/ are called semivowels because their sound quality is very much like vowels in that there is no obstruction when producing the sounds. But they are not full vowles since they cannot form a syllable by themselves. For this reason, semivowels are treated as consonants. They occur in front of vowels and glide into vowels and  thus are also called glides. In other words, semivowels cannot form the nucleus of a syllable, and can occur only in front of a vowel in a syllable. When a vowel sound follows a vowel within a syllable, the combination is viewed as a diphthong, which is a phoneme made of two vowel sounds. So, for instance, the ‘w’ and ‘y’ in “wife” and “year” are glides, but those in “owl” and “boy” are not. The latter are viewed as parts of diphthongs /aʊ/ and /ɔɪ/. 

Semivowels vs their vowel counterparts, /u/ and /i/ 

In light of the quality of the sound, /w/ and /j/ are less-sonorous equivalents of their counterpart vowel sounds, /u/ and /i/. When we say /w/ or /j/ slowly, we can hear the vowel sounds /u/ and /i/, respectively: woo, yee.  While sounding similar, there are sufficient differences between the two pure vowel sounds and the semivowels. /w/ is a labial sound, as it is produced at the lips. By contrast, /u/ is a back vowel as it is produced in the back of the mouth. Regarding /i/ and /j/, /j/ is a palatal consonant, being produced at the center of the mouth. By contrast, /i/ is a frontal sound, being produced at the front of the mouth. Also, the tongue for /j/ is placed closer to the roof of the mouth than the vowel /i/. With this introduction, let’s examine each glide in detail beginning with /w/.