A locking system for mechanical joining of floorboards (1, 1′) which have a body (30), a lower balancing layer (34) and an upper surface layer (32). A strip (6) is integrally formed with the body (30) of the floorboard (1) and extends under an adjoining floorboard (1′). The strip (6) has a locking element (8), which engages a looking groove (14) in the underside of the adjoining floorboard (1′) and forms a horizontal joint. A tongue (38) and a tongue groove (36) form a vertical joint between upper and lower plane-parallel contact surfaces (43, 45) and are designed in such manner that the lower contact surfaces (45) are on a level between the upper side of the locking element (8) and a plane containing the underside (3) of the floorboard. Also, a floorboard having such a locking system, a floor made of such floorboards, as well as a method for making such a locking system.