Prefix · joint action / together
Signals joint action or union: cooperate, combine, connect, coexist, coworker.
Written by Bryan López, English teacher · Updated
Co- (and its variants com- and con-) comes from Latin "cum" (with, together) and signals joint action, collaboration, or union. cooperate = work together with others; coexist = exist alongside another; combine = join two or more things; connect = link points together. The core idea is always "more than one acting or being together."
The variant depends on the first letter of the root: co- is the most general and modern form (cooperate, coexist, coauthor), com- appears before "p," "b," or "m" (combine, compete, compress), and con- before consonants in general (connect, control, confirm). Co- is the most productive variant in modern English for creating new compound words: co-founder, co-host, co-pilot.
The variant depends on the first letter of the root:
Co- is the most visible and productive variant in modern English: co-founder, co-host, co-pilot. Com- and con- appear almost exclusively in already-established words.
co-/com-/con-/koʊ/, /kɒm/, /kɒn/
Tap the button to hear how the prefix sounds. Each word in the table has its own audio.
"Coworker" and "co-worker" are both correct. American English tends to drop the hyphen; British English keeps it more often.
"Cooperate" implies active, deliberate collaboration. It is also used in formal and legal contexts: "the suspect cooperated with police."
A trick to remember it
Co- is the prefix of teamwork. Whenever you see co-, com-, or con- at the start of a word, ask whether there is an idea of "together" or "jointly": cooperate, combine, connect, coexist, coworker. The Latin root "cum" (with) runs through all of them.
Exercise 1 · Form the word
Exercise 2 · Pick the right one
"The two rival companies decided to ___ on the research project to share costs and expertise."
Exercise 3 · Pick the right one
Which word does NOT use co-/com-/con- with the sense of "together or union"?
The prefix co-/com-/con- signals joint action or union: cooperate, combine, connect, coexist, coworker. In Spanish it usually maps to co- / con-.
The prefix co-/com-/con- is pronounced /koʊ/, /kɒm/, /kɒn/. For example, "cooperate".
"operate" becomes "cooperate". It is a typical example of the co-/com-/con- prefix.
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