Rectangular floorboard

The invention relates to a locking system for mechanical joining of floorboards (1) along a vertical joint plane (F), and a floorboard comprising such a locking system. For joining perpendicular to the joint plane (F) there is a locking groove formed in the underside (3) of a first joint edge at a distance from the joint plane (F), and a portion (P) projecting from the lower part of the second joint edge (4a) and below the first joint edge and, at a distance from the joint plane (F), supporting a locking element (8) cooperating with the locking groove (14), said projecting portion (2) thus being located entirely outside the joint plane (F). The projecting portion (P) presents at least two horizontally juxtaposed parts (P1, P2), which differ from each other in respect of at least one of the parameters material composition and material properties.