groove system in another embodiment the second pair of opposing joint edges are provided with a mechanical locking system which only provides locking in the horizontal direction such as would be the case if the tongue 38 of any one of the embodiments of fig2 b or 2 c was to be removed while leaving the locking strip 6 with its locking element 8 and the locking groove 14 in fig2 d such a case would be accomplished if the tongue 38 or the lower lip 39 will be removed fig3 a c illustrate a floorboard which in this embodiment has a core 30 of a relatively soft material such as mdf or particle board the locking system has been adjusted to the soft core by the locking element 8 having a horizontal extent which is about 0 5 times the thickness of the core 30 the surface layer 31 has outer joint edges 40 41 which in this embodiment project beyond the outer parts of the core 30 this projection can be some tenths of a millimeter the outer parts of the surface layer are pressed together in connection with laying and the floorboards will have very tight joints the mechanical locking system guides the floorboards in exact positions and ensures a high quality of laying in one embodiment the locking system may have a geometry where a play may exist between the locking surface 9 of the locking element.