Abstract: An upper piggybacked unit of a tandem unit is provided with a hinge unit and at least one motion lock to prevent the upper unit from falling away from the lower unit when it is tandemly coupled to the lower unit and the lower unit is pivoted to a non-horizontal position. The motion lock may inhibit rotation and/or translation of the upper unit. The motion lock may also be self-actuating, such as a gravity dependent lock mechanism. A lock override mechanism may be provided to allow the lock to yield to excessive force without causing damage. The lock enable condition may be automatically reset after the override condition so that no special knowledge or action on the part of the user is required to return the hinge to normal operation.