399. Words formed directly from a verb stem are called verbals; those formed from a noun or adjective stem are called denominatives.
b. As in conjugation (cp. §§ 322 and a) a -σ-, of uncertain origin, is sometimes added to a verb stem before a suffix. So also -θ- in στα-θ-μός (station) from στα- (stand), and in ῥυ-θ-μός (rhythm) from ῥέω (ῥυ-) (flow).
c. A final stem vowel and the vowel of a suffix often contract.
d. A final vowel of a stem is sometimes dropt before a vowel.
f. A final consonant is sometimes dropt before the ending.