com- Also co-, col-, con- and cor-.
[Latin cum, with.]
Which form is used depends on the initial letter of the stem: com- is usual before b, m, and p, some vowels, and sometimes f (combine, commerce, compact, comestible, comfort); co- before vowels, h, and gn (coerce, cohabit, cognate); col- before l (collect); cor- before r (correct); and con- before other consonants (concede, connect, contain). Many words of these types were created in Latin and brought over into English; in many of them the form has the effect of intensifying the root word. The form co- often makes a hyphenated pair with another English word, indicating some activity taken in common or jointly: co-author, co-driver, co-education, co-founder, co-pilot, co-producer, co-purchaser.