Abstract: Apparatus for locking and holding structures to one another in a releasable manner, including a separable element which may be fastened to each of the structures. One of the elements may be a generally spherical member, similar to the ball fixture utilized in cooperation with a trailer hitch. The other fixture includes a housing with a dual-acting or compound motion mechanism pivotally mounted therein. The mechanism may comprise a gate having a locking surface which cooperates with a locking seat in the housing to receive and lock the spherical element therebetween. A locking mechanism may be movably mounted on the gate to limit the movement of the latter as well as to fix it in the locking position.