A building block comprising: (a) a body having opposite first and second sides (174, 176), opposite first and second end faces (178, 180) extending between the first and second sides (174, 176), and opposite upper and lower faces extending between the first and second sides (174, 176) and the first and second end faces (178, 180); (i) the upper and lower faces being planar and parallel to each other; (b) a leg extending from the first side of the body; (i) the leg having opposite leg sides (194, 195) and a leg end face; (ii) the leg sides (194, 195) extending between the first side of the body and the leg end face; (iii) the first side of the body having a first portion extending between the first end face and one of the leg sides (194, 195); (iv) the first side of the body having a second portion extending between the second end face and the other of the leg sides (194, 195); (v) a widest portion of the leg being at the leg end face; characterized in that (vi) the leg end face has a first section recessed inwardly from one of the leg sides (194, 195) at an angle and a second section recessed inwardly from the other of the leg sides (194, 195) at an angle which is the same as the angle of the first section. (a) a body having opposite first and second sides (174, 176), opposite first and second end faces (178, 180) extending between the first and second sides (174, 176), and opposite upper and lower faces extending between the first and second sides (174, 176) and the first and second end faces (178, 180); (i) the upper and lower faces being planar and parallel to each other; (i) the upper and lower faces being planar and parallel to each other; (b) a leg extending from the first side of the body; (i) the leg having opposite leg sides (194, 195) and a leg end face; (ii) the leg sides (194, 195) extending between the first side of the body and the leg end face; (iii) the first side of the body having a first portion extending between the first end face and one of the leg sides (194, 195); (iv) the first side of the body having a second portion extending between the second end face and the other of the leg sides (194, 195); (v) a widest portion of the leg being at the leg end face; characterized in that (vi) the leg end face has a first section recessed inwardly from one of the leg sides (194, 195) at an angle and a second section recessed inwardly from the other of the leg sides (194, 195) at an angle which is the same as the angle of the first section. (i) the leg having opposite leg sides (194, 195) and a leg end face; (ii) the leg sides (194, 195) extending between the first side of the body and the leg end face; (iii) the first side of the body having a first portion extending between the first end face and one of the leg sides (194, 195); (iv) the first side of the body having a second portion extending between the second end face and the other of the leg sides (194, 195); (v) a widest portion of the leg being at the leg end face; characterized in that (vi) the leg end face has a first section recessed inwardly from one of the leg sides (194, 195) at an angle and a second section recessed inwardly from the other of the leg sides (194, 195) at an angle which is the same as the angle of the first section.