Abstract: A concrete block is provided having half dovetail sections at opposite ends of the block arranged so that when placed together the dovetailed sections on adjoining blocks form a full dovetailed section which is just large enough to be received within a dovetail groove in the central portion of an adjacent identical block. The concrete blocks are provided with upper and lower longitudinal grooves for longitudinal members having the same exterior shape. In one form of the invention one side of the block may be made into a plant receptacle by providing a hollowed out section oriented upwardly. Insulating concrete blocks may also be combined with blocks formed of structural concrete to provide an interlocked wall having a portion of the wall made from insulated concrete material and another portion of the wall made from structural concrete material.