com-, co-, cog-, col-, con-, cor-
(Latin: together, together with, with)
The prefix com- is assimilated to co- before h, w, and all vowels:
The prefix com- becomes, cog- before g: cognition, [co + gnoscere, "to know"], et al.
The prefix com- becomes, col- before l: colloquial, et al.
The prefix com- becomes, con- before c, d, g, j, n, q, s, t, v: covivant, et al.
The prefix com- becomes, cor- before r: corrigible, et al.