Source : Webster's Revised Unabridged Dictionary (1913)

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.