Preclude means to rule something out in advance — to make it impossible before it can happen: "the injury precluded any return to play." Exclude means to leave someone or something out of a group: "the price excludes VAT." Preclude acts on possibilities; exclude acts on members of a set.
The core difference
- preclude — makes an outcome impossible, usually in advance: Nothing in the contract precludes a second bid.
- exclude — leaves something out of a group or total: The figure excludes overheads.
What each takes as its object
This is the reliable test. Exclude takes a thing or person that could belong to a set. Preclude takes an action or possibility. "The rule excludes members under 18" removes people; "the rule precludes members under 18 from voting" blocks an action.
Origins that hold the distinction
Both share Latin claudere, to shut. Prae- is "before", so preclude is to shut off beforehand. Ex- is "out", so exclude is to shut out. The prefixes still describe exactly what each word does.
A common misuse
"Excluded from happening" is a slight muddle; possibilities are precluded, not excluded. It rarely obscures meaning, but preclude is the tighter word.
