the invention relates to a locking system for mechanical joining of floorboards constructed from a body , a rear balancing layer , and an upper surface layer . a strip , which is integrally formed with the body of the floorboard and which projects from a joint plane and under an adjoining board , has a locking element which engages a locking groove in the rear side of the adjoining board . the joint edge provided with the strip is modified with respect to the balancing layer , for example by means of machining of the balancing layer under the strip , in order to prevent deflection of the strip caused by changes in relative humidity . the invention also relates to a floorboard provided with such a locking system , as well as a method for making floorboards with such a locking system .