[...] (X stands for compact grave consonants, S, for noncompact acute consonants, F, for noncompact grave consonants other than noninitial {*"m"}) [...].
- Halle (2004), Pag. 25

Except after noninitial {*"m"} there can be n sequences of grave noncompact (labial) consonants.
- Halle (2004), Pag. 25