Grandfather clause
Etymology
The term has a dark origin: after the U.S. Civil War, several Southern states passed “grandfather clauses” letting men vote only if their grandfathers had — a device to disenfranchise Black citizens while exempting whites from new literacy and property tests. The Supreme Court struck these down in 1915, but the phrase survived for any exemption of the pre-existing.
noun phrase · /GRAND-fah-ther klawz/
A provision that exempts existing cases from a new rule, letting them continue under the old terms.
What it names
The common compromise by which a new regulation applies only going forward, sparing those already in place.
Example
The new building code required sprinklers, but older houses were grandfathered in.
Related idea
A grandfather clause exempts what already exists from a new standard; ex post facto law runs the opposite and forbidden way, punishing acts that were legal when they were done.