Floorboards provided with a locking system for mechanical joining of said floorboards (1, 1') having a core and opposite first and second joint edge portions (4a, 5a and 4b, 5b, respectively), adjoining floorboards (1, 1') in the mechanically joined position having their first and second joint edge portion (4a, 5a and 4b, 5b, respectively) joined at a vertical joint plane, said locking system comprising a) mechanical cooperation means (36, 38) for vertical joining of the first joint edge portion of a first floorboard of said floorboards and the second joint edge portion (4a, 5a and 4b, 5b, respectively) of an adjoining second floorboard of said floorboards, and b) mechanical cooperating means (6, 8, 14) for horizontal joining of the first and second joint edge portions (4a, 5a and 4b, 5b, respectively), the mechanical cooperating means (6, 8; 14) comprising a locking groove formed in the underside of said second floorboard and extending parallel with and at a distance from the vertical joint plane at said second joint edge portion (4b, 5b) and having a downward directed opening, and a strip integrally formed with the core of said first floorboard, which strip at said first joint edge portion (4a, 5a) projects from said vertical joint plane and at a distance from the joint plane has a locking element which projects towards a plane containing the upper side of said first floorboard and which has at least one operative locking surface for coaction with said locking groove, the locking groove, seen in the plane of the floorboards and away from the vertical joint plane, having a greater width than said locking element, wherein the locking system is configured such that the strip is bendable downwards to enable taking up of an installed floorboard by upward angling, wherein said at least one operative locking surface of the locking element is essentially plane and located at the upper part of the locking element at a distance from the upper side of the projecting strip and faces the joint plane, wherein the locking groove has at least one essentially plane operative locking surface which is located in the locking groove at a distance from the opening of the locking groove and which is designed to cooperate with said locking surface of the locking element in the joined position, wherein the locking groove at the lower edge closest to the joint plane has an inclined or rounded guiding part which extends from the locking surface of the locking groove and to the opening of the locking groove and which is intended to guide the locking element into the locking groove by engaging a portion of the locking element which is positioned above the locking surface of the locking element or adjacent to its upper edge, and characterised in that the operative locking surfaces (10 and 11, respectively) of the locking element and the locking groove make a locking angle of at least 80° to the upper side of the boards (1, 1') in clockwise direction a) mechanical cooperation means (36, 38) for vertical joining of the first joint edge portion of a first floorboard of said floorboards and the second joint edge portion (4a, 5a and 4b, 5b, respectively) of an adjoining second floorboard of said floorboards, and b) mechanical cooperating means (6, 8, 14) for horizontal joining of the first and second joint edge portions (4a, 5a and 4b, 5b, respectively), the mechanical cooperating means (6, 8; 14) comprising a locking groove formed in the underside of said second floorboard and extending parallel with and at a distance from the vertical joint plane at said second joint edge portion (4b, 5b) and having a downward directed opening, and a strip integrally formed with the core of said first floorboard, which strip at said first joint edge portion (4a, 5a) projects from said vertical joint plane and at a distance from the joint plane has a locking element which projects towards a plane containing the upper side of said first floorboard and which has at least one operative locking surface for coaction with said locking groove, the locking groove, seen in the plane of the floorboards and away from the vertical joint plane, having a greater width than said locking element, a locking groove formed in the underside of said second floorboard and extending parallel with and at a distance from the vertical joint plane at said second joint edge portion (4b, 5b) and having a downward directed opening, and a strip integrally formed with the core of said first floorboard, which strip at said first joint edge portion (4a, 5a) projects from said vertical joint plane and at a distance from the joint plane has a locking element which projects towards a plane containing the upper side of said first floorboard and which has at least one operative locking surface for coaction with said locking groove, the locking groove, seen in the plane of the floorboards and away from the vertical joint plane, having a greater width than said locking element,