Abstract: A building block (10) of the type which can be used to construct a structure made up of a number of identical such blocks disposed side to side. The blocks are usually square although triangular blocks may be used in conjunction therewith. The preferred form of the square block is provided with four flat sides including parallel first (14) and second (18) sides and parallel third (16) and fourth (12) sides. Two flanges (42, 40) extend along the respective first and third sides, projecting outwardly therefrom. Two recesses (60, 49) shaped complementarily to the flanges extend along the respective second and fourth sides, projecting inwardly therefrom. The first flange intersects the second flange adjacent the junction between the first side and the third side. At their opposite ends the flanges comprise triangular portions (54, 66) which span the respective recesses and are provided with indentations in their outer faces which are aligned with the recesses.