The K-behaviour of p-trees.

For use in combination with a box-type receptable containing electrical contacts such as switch contacts or a socket, a first cover comprising a planar frame on which the cover is removably mounted, the frame defining an access opening, a pair of fixed spaced headboards extending across the frame, the headboards being at opposite ends of the opening and having their outer surfaces lying substantially in a plane that is disposed slightly outwardly from the plane of the frame and defining between them a space that extends over the access opening and transversely entirely across the frame, a complementary cover mounted in and occupying that space, the complementary cover substantially entirely filling the space and extending transversely entirely across the space and having on opposite end edges sliding fit with the headboards, with the outer surface of the complementary cover substantially coplanar with the outer surfaces of the headboards, and pins on the inner side of the complementary cover coacting with recesses on the frame for removably attaching the complementary cover to the first cover to overlie the cover opening.