Exception that proves the rule

“The exception [that] proves the rule” is a saying whose meaning has been interpreted or misinterpreted in various ways. Its true, or at least original, meaning is that the presence of an exception applying to a specific case establishes (“proves”) that a general rule exists. For example, a sign that says “parking prohibited on Sundays” (the exception) “proves” that parking is allowed on the other six days of the week (the rule). A more explicit phrasing might be “the exception that proves the existence of the rule.”