An important class of proto-sounds were those called resonants (or sonants). The phonetic property of resonants which set them apart was that they behaved like consonants [m n r l w y] or vowels [m. n. r. l. u i] depending on their environment in a syllable. (N.B. On this page, we use '.' after a resonant letter to signal what would ordinarily be rendered as ring below the letter -- signifying vocalic value.)
In English words such as 'run, race' or 'not, now', the resonants /r/ and /n/ are consonantal; but in 'butter' and 'button', where the vowel of the last syllables is unstressed and disappears, the final resonants becomes vocalic: [but-r.] and [but-n.].
As the vowel of old roots with consonantal enlargements and suffixes of CVC-C- was reduced to CC-C-, or CC-VC, where one of the root consonants was a resonant, the resonant as syllable nucleus would function as a vowel.
The PIE root for 'water' may be rendered with initial resonant *w, and in the root form *wed- = *wet'- (CVC) where *w is followed by a vowel, the resonant (glide) has the consonantal value [w]; but it can also have its counterpart vocalic value [u] as in the root variant *wd- or *ud- (CC-). Sanskrit ud-n-ás 'of water' reflects the situation where *w is vocalic.