Abstract: An end section for a rail which is engageable with a post having a slot. The end section is so arranged that in one orientation the end section is releasable from a slot of the post, and when rotated to a second orientation, the end section is in a locked position and cannot be released from the slot of the post. The end section may be fastened, or otherwise attached to a rail and used in a demountable enclosure or support structure having at least two posts. Each post of the demountable enclosure or support structure has at least one slot for receiving a rail and the at least one rail is releasably mountable between an adjacent pair of the posts. The end section of the rail is engageable in a slot provided in one of the adjacent posts, and each slot is complementary, at least in part, to the end section of a rail. Each rail, when mounted to an adjacent post, is moveable between a locked position and an unlocked position by rotation of the respective rail.