Com- \Com-\ A prefix from the Latin preposition cum, signifying with, together, in conjunction, very, etc. It is used in the form com- before b, m, p, and sometimes f, and by assimilation becomes col- before l, cor- before r, and con- before any consonant except b, h, l, m, p, r, and w. Before a vowel com- becomes co-; also before h, w, and sometimes before other consonants.

Source: Webster's Revised Unabridged Dictionary (1913)