A constructional building set comprising gear boxes which are provided with guide grooves and shaft passages. The guide grooves and the shaft passages are constructed such that transmission elements can be inserted into the guide grooves from the outer side of the gear box in such a manner that the transmission elements are positioned in alignement with the shaft passages in the gear boxes. When the gear box additionally comprises means for releasable geometrical retention of the transmission elements in their position in alignment with the shaft passages after insertion of the transmission elements into the gear box, it is ensured that the shafts can be pulled out of the assembled gear structure without the transmission elements dropping out of the gear structure. This simplifies the mounting operation and makes it easier to use the gear structure as an assembled unit that can be used for different applications.