Abstract: The set of blocks in this invention comprise "slot blocks" and "key blocks." A slot block is a cube having two slots on each face. Each slot crosses the entire face and bisects a pair of opposite edges. The cross section of each slot is a square whose width and depth are one-fifth the length of the edge of the cube. A key block is a rectilinear polygon whose thickness is equal to the slot width and depth. Each side of each polygon has a length that is a whole number multiple of the slot width and depth. The shapes of the key blocks are such that they can completely fill all the slots, internal and external, of an aligned stack of slot blocks, with the external key blocks forming continuous intersecting ridges whose width and height are equal to the slot width and depth. The key blocks, with or without the slot blocks, also lend themselves to the construction of other attractive forms that would be rarely, if ever, realized with other block sets.