The flat bottom piece on a post enabling the post to be mounted on top of an existing surface. 2″ x 2″ posts with welded plates have a 4″ x 4″ x 1/4″ metal plate with 4 pre-drilled holes. 2.5″ x 2.5″ posts with welded plates have a 6″ x 6″ metal plate with 4 pre-drilled holes. 3″ x 3″ posts with welded plates have a 6″ x 6″ metal plate with 4 pre-drilled holes. Anchor bolts are not included; purchase them locally based upon the type of material you are mounting your posts to.