Floorboards with substantially planar and parallel upper top sides and lower undersides and panel material located between the upper and lower top sides, and a mechanical locking system for locking a first edge of a first floor board to a second edge of a substantially identical second floor board, the mechanical locking system including a tongue and groove forming a first mechanical connection locking the first and second edges to each other in a first direction at right angles to a principal plane of the floor boards, and a locking device forming a second mechanical connection locking the first and the second edges to each other in a second direction and at right angles to the edges; wherein, when the first edge and the second edge are locked together, there is space in the locking system between the first and the second edges.