A block used to construct a wall. The block has a front and rear surfaces, top and bottom surfaces, and side surfaces. The bottom surface of the block has a projection that extends downwardly therefrom and which is configured to engage one or more blocks in an adjacent course of blocks to prevent movement therebetween. Depending upon the configuration of the wall, the projection may engage a portion of the rear surface of an adjacent course of blocks, or may be received within one of two channels in the top surface of the adjacent course of blocks. By selecting the position of the projection relative to the adjacent course of blocks, differently configured walls may be constructed.