Abstract: A building block system, in particular a toy building block system, comprises a plurality of inter-connectable building blocks (1), whereby each building block (1) comprises an undercut trough-shaped groove (3) on at least one lateral surface (2) into which a connection bar (5) with a cylindrical part (6)is insertable. The building block system is suited for a variety of uses, easy to handle and provides many form-locking connection options of one or several building blocks, including of building blocks of differing geometrical form. According to the invention, the connection bar (5) is also formed with a hammer head-shaped boundary body (7) on at least one end of the cylindrical part (6), which can be inserted into a corresponding recess (11) on the end of the groove (3) in the building block (1) so as to form-lock with the lateral surface (2) of the same, when the connection bar (5) snaps into the groove (3) of the building block (1).

Abstract: A plug-in module is described; one side of this incorporates four spring leaves (6, 6') that can be deflected slightly outwards and which are arranged on one side in a tulip shape and which, on the other five sides surfaces (2 to 5), can be snapped onto a similarly constructed plug-in module. The side surfaces incorporate ridges (8, 8') which can be either continuous or segmented. On their inner sides, the spring leaves are provided with corresponding connecting elements (7, 7', 7") which snap behind the ridges described above. The production of a connection between two plug-in modules is effected first by a slight outward deflection of the spring blades from their rest position and by them subsequently snapping into position behind the ridges described above. The plug-in modules entail the advantage that even after a long time and frequent use, they can still be connected to each other easily or released from each other with equal ease.

Abstract: In a wheel for use with a modular assembly system with plug-in-type elements, the dimensioning of all of the elements being based on a basic unit length a, the wheel having a square central axle hole with a cross-section of a.times.a, rectangular slots with a width a being radially cut into both of the laterial faces of the wheel and arranged in pairs axially opposite to each other, axial openings are provided in the areas of the slots to provide a connection between the axially opposed slots.