the exception that proves the rule

the exception that proves the rule

You say that something is the exception that proves the rule to mean that the example that you have just mentioned is not normal and is the opposite of what you usually find. Towers should generally be arranged in clusters, but the Post Office Tower was the exception that proved the rule — it needs to stand alone so that its signals are not interrupted. The most creative minds are often said to be the product of a problematic childhood, but Hornby must be the exception that proves the rule. Note: `Prove' here means `to test by experiment or analysis' rather than `to establish as true'. So, the meaning is that an exception tests a rule, not that it establishes the rule as true in all other situations.
See also: exception, prove, rule, that

the exception that proves the rule

a particular case that is so unusual that it is evidence of the validity of the rule that generally applies.
This phrase comes from the Latin legal maxim exceptio probat regulum in casibus non exceptis ‘exception proves the rule in the cases not excepted’. This in fact meant that the recognition of something as an exception proved the existence of a rule, but the idiom is popularly used or understood to mean ‘a person or thing that does not conform to the general rule affecting others of that class’
1998 Spectator The success of The Full Monty in the United States is an exception which proves the rule. On such lucky breaks, industries and economies are not built.
See also: exception, prove, rule, that

the exˈception that proves the ˈrule

(saying) people say that something is the exception that proves the rule when they are stating something that seems to be different from the normal situation, but they mean that the normal situation remains true in general: English people are supposed to be very reserved, but Pete is the exception that proves the rule — he’ll chat to anyone!
See also: exception, prove, rule, that