Com-

CIDE DICTIONARY

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.  [1913 Webster]

OXFORD DICTIONARY

Com-, prefix (also co-, col-, con-, cor-) with, together, jointly, altogether.

Usage

com- is used before b, m, p, and occas. before vowels and f; co- esp. before vowels, h, and gn; col- before l, cor- before r, and con- before other consonants.

Etymology

L com-, cum with