The exception affirms the rule to be the other way.

Explanation

(English-origin maxim.) The very existence of an exception demonstrates that the rule runs the other way for all cases not excepted — the exception proves the affirmative rule by marking its boundary.

Where this applies

• A statute says "No vehicles in the park, except emergency vehicles" — the exception for emergency vehicles proves the general rule that all other vehicles are banned.
• A lease says "No pets allowed, except service animals" — the exception for service animals affirms the rule that ordinary pets are prohibited.
• A sign says "Employees only beyond this point, except during public tours" — the exception for tours proves that at all other times, only employees may enter.