But when a relative key is on the opposite side of the circle from its home key , then in theory modulating to that key would involve adding so many sharps or flats that double-sharps and double-flats would have to be written in the key signature . The following six keys ( which are the parallel major\/minor keys of those above ) would require one , two or three double-sharps or double-flats : Such a key is one whose key signature would contain one or more double-flats or double-sharps .