Some modern writers make a distinction between the sound of u, when it follows r, as in rude, truth, and its sound when it follows other letters, as in mute, duke; making the former sound equivalent to oo; rood, trooth; and the latter a diphthong equivalent to eu or yu. This is a mischievous innovation, and not authorized by any general usage either in England or the United States. The difference, very nice indeed, between the sound of u in mute, and in rude, is owing entirely to the articulation which precedes that letter. For example, when a labial precedes u, we enter on its sound with the lips closed, and in opening them to the position required for uttering u, there is almost necessarily a slight sound of e formed before we arrive at the proper sound of u. When r precedes u, the mouth is open before the sound of u is commenced. but in both cases, u is to be considered as having the same sound.
In some words, as in bull, full, pull, the sound of u is that of the Italian u, the French ou, but shortened. This is a vowel.
U has another short sound, as in tun, run, sun, turn, rub. This also is a vowel.
