exception proves the rule

Interestingly, the old (some would say \"real\") meaning of this phrase is
something like \"the exception proves that there *is* a rule.\"

The common misconception is that \"proves\" in this phrase means \"tests\".
That is *not* the case, although \"proof\" *does* mean \"test\" in such
locutions as \"proving ground\" and \"The proof of the pudding is in the
eating.\"

The Dictionary of Modern English Usage explains this saying as follows:

\"'Special leave is given for men to be out of barracks tonight till 11.0
p.m.'; 'The exception proves the rule' means that this special leave
implies a rule requiring men, except when an exception is made, to be in
earlier. The value of this in interpreting statutes is plain.\"