CREATE TABLE table_37212 ("Name" TEXT, "Birth" TEXT, "Marriage" TEXT, "Became Duchess" TEXT, "Ceased to be Duchess" TEXT, "Death" TEXT, "Spouse" TEXT)

Which Marriage has a Became Duchess of 12 december 1666?
SELECT "Marriage" FROM table_37212 WHERE "Became Duchess" = '12 december 1666'
SELECT "Marriage" FROM "table_37212" WHERE "Became Duchess" = '12 december 1666'