return this being respectively an upper longitudinal one 5 a a lower longitudinal one 5 b a vertical one 5 c and a vertical one 5 d in the embodiment shown in detail in this fig4 the respective horizontal 5 b and vertical 5 d returns are each associated with a fin 6 to form a v section or u section rebate 7 facing outward and able to receive or fit over the catching tabs formed by the respective horizontal return 5 a of the wall of a block in the course beneath and vertical return 5 c of the wall of a juxtaposed block in the course that is being laid the web 2 a of each wall 2 has passing through it catching openings 8 which being in the shape of buttonhole slots are arranged with the same spacing near its respective upper and lower edges fig1 to 3 show that each wall 2 has on its external face vertical grooves 19 which forming frangible thinnings mark out the regions at which the block can be cut in order to adapt its length to suit the building need in the example shown there are three grooves per wall these being arranged mid way along the block and near its ends but the number and distribution of such grooves may differ the web 2 a of the wall also comprises projecting from its face that faces inward vertical ribs.