The building block includes upper and lower sides having longitudinally extending rib and groove respectively, interfitting with the groove and rib of a compatibly formed block. The rib and groove include laterally spaced faces inclined in the same general direction and interconnected by a transverse flat face. One of the inclined faces of the rib and of the groove is provided with a stepped shoulder, the stepped shoulder of the rib face engaging the stepped shoulder of the groove face of a compatible block to preclude relative movement of the said inclined rib and groove faces having the stepped shoulders tending to separate the rib and groove.