Floorboards provided with a mechanical locking system including a locking strip protruding from a first edge of a first floorboard. The locking strip is provided with a locking element configured to cooperate with a locking groove at a lower side of a second edge of a second floorboard for locking the first edge and the second edge in a horizontal direction. The first edge and the second edge are configured to be assembled by a vertical downward motion of the second edge towards the first edge. The second edge is provided with a calibrating groove adjacent the locking groove. Also, a method for producing a mechanical locking system.