Semi-consonants (aka semi-vowels or glides) are sounds that are articulated with the tongue higher than for a high vowel, but not so high as to actually touch the palate. Note also that semi-consonants are shorter than actual vowels. The most important distinguishing feature of semi-consonants is that, unlike vowels, they cannot be at the centre of a syllable. French uses three semi-consonants: /j/ , /ɥ/ and /w/ . Semi-consonants behave sometimes like a vowel and sometimes like a consonant. Note that, since they cannot be the centre of a syllable, any time there is a semi-consonant in a syllable there must also be a vowel.
Generally speaking (and there are some exceptions), semi-consonants are found in front of vowels, especially where one would otherwise find a high vowel (i.e.: /i/, /y/ or /u/) followed by another vowel. In other words, French avoids the sequence "high vowel + vowel". For example, while the word piano is written with "i", it does not contain the high vowel /i/. Rather, it contains a /j/, i.e.: [pjano] .
The following sections consider each of the three semi-consonants found in French.