The sounds of contemporary American English (as of all Englishes) fall into five classes: obstruents (= “true” consonants), vowels, liquids, nasals, and glides. The latter four are called sonorants. By contrast to the obstruents, they are “vowel-like” in virtue of having significantly greater sonority. The glides (sometimes called “semi-vowels”) are comprised by the sounds /w, j, h/ and are of special interest in English because these sounds are defined as being neither consonantal nor vocalic. They behave more like consonants than vowels but are not “true” consonants because of their definition phonologically as non-consonantal. They are thus outliers in the system of English sounds.
This marginal phonological status is mirrored iconically by their preponderance in the sound structure of the word class where they typically occur, viz. interjections. An interjection––unlike verbs, nouns, and generally words proper, which have a referential function––has only a phatic and/or a conative function. It is thus at the functionally restricted end of the scale of word classes, just as are the glides phonologically. Here we have a case of linguistic iconicity that affects the entire system of sound-sense alignments that makes language a coherent structure and not merely an aggregate.