The Collaborative International Dictionary
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. [1913 Webster] ||
Douglas Harper's Etymology Dictionary
word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.\n
\nBefore vowels and aspirates, reduced to co-; before -g-, assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, -v- assimilated to con-.
Wiktionary
pre. the form of con- used before ''b'', ''m'', and ''p''
Usage examples of "com-".
For the record, I remind my fellow members that the Special Counsel was chosen without consultation with my office or Com- Mine Station.
Director Dios, both my office and Com- Mine Station have been questioned as rigorously as I hope you will be questioned now.