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exception proves the rule, the

exception proves the rule, the idiom

An instance that does not obey a rule shows that the rule exists. For example, John's much shorter than average but excels at basketball—the exception proves the rule. This seemingly paradoxical phrase is the converse of the older idea that every rule has an exception. [Mid-1600s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.