of troy, Reed is correct on this one. From the OED:

grandfather clause U.S. colloq., a clause in the constitutions of some Southern states, exempting from suffrage restrictions the descendants of men who voted before the Civil War;


it has displaces other common laws... there are still exceptions--even in US.

In Canada, Québec uses French Civil law, and on this pretext is guaranteed three members on the nine member Supreme Court of Canada.