A locking system for mechanical joining of floorboards. For horizontal mechanical joining there is a projecting portion with a locking element which cooperates with a locking groove in an adjacent board. A tongue-and-groove joint for vertical mechanical joining has cooperating upper abutment surfaces and cooperating lower abutment surfaces which are essentially parallel with the principal plane of the floorboards and of which the lower abutment surfaces are positioned essentially outside the outer vertical plane, i.e. displaced relative to the upper abutment surfaces. The tongue is movable at an angle into the groove and the locking element is insertable into the locking groove by mutual angular movement of the boards about the joint edges.