An assembly unit (1) particularly suited for a toy, but which may also be used for decoration and hobby use and other purposes. The particular advantage of the unit is that it may be assembled with any number of identical units (1), thereby creating coherent, continuous planes and three-dimensional structures. This is achieved by the assembly unit being shaped as a double cross with an elongated rectangular trunk (2), the width of which is one-fifth of the length of the trunk. On the trunk (2) is positioned four side branches (3), in pairs on each side of the trunk, at a distance equal to the width of the trunk from the ends (6) of the trunk, thereby forming an opening (7) on each side of the trunk, with a width equal to the width of the trunk. In a particularly expedient embodiment the assembly unit (1) has a height equal to the width of the trunk. This makes possible the assembly of very stable threedimensional combinations of structures, especially if the units are made of a resilient material such as blow molded polyethylene. The assembly units can be used for an assembly set consisting exclusively of assembly units of identical shape, but complementary assembly units (11, 12) are also possible. The assembly units may also be made of hard materials such as tile, brick and concrete to serve other purposes.