Any clause in a law which makes an exception for people or
things if, when the law was passed, they were already doing
whatever is being forbidden. And related concepts. Building
codes often grant exceptions for buildings built before the code
was enacted, for example.  The phrase comes from a series of
similar changes in the constitutions of six southern US states
between 1895 and 1910. The right to vote was limited to literate
people with some arbitrary amount of wealth, but exceptions were
made for people who'd had the right to vote during the Civil
War, or whose ancestors had had that right at the time. By
chance, quite coincidentally, the set of people allowed to vote
in those states during the Civil War was the set of all white
people in those states, and both poverty and illiteracy were
depressingly common among black and white people both. The
effect, perhaps unintentional, was to limit the franchise to
most whites (not counting poor, illiterate immigrants) and a
minority of blacks.  The United States