A locking system for mechanical joining of floorboards has a locking groove which is formed in the underside of and extends in parallel with the first joint edge at a distance from the joint plane, and a portion projecting from the lower part of the second joint edge and below the first joint edge and integrated with a body of the board. The projecting portion supporting at a distance from the joint plane a locking element cooperating with the locking groove and thus positioned entirely outside the joint plane seen from the side of the second joint edge, the projecting portion having a different composition of materials compared with the body of the board. The projecting portion presents at least two horizontally juxtaposed parts, which differ from each other at least in respect of the parameters material composition and material properties.