A seat comprising: - a seat frame comprising at least a first crosspiece and a second crosspiece positioned at a distance from the other and delimiting a seat space, - a seat made of stretchable material and shaped to at least partially cover the seat space delimited by the crosspieces (61, 62) of the seat frame, the seat comprising at least a first edge and a second opposite edge, the first edge being positioned on the first crosspiece of the seat frame and the second edge being positioned on the second crosspiece of the seat frame, the seat comprising at least one tensioning device comprising: - at least one stiffening member arranged on the first edge, the stiffening member extending at least partially along the first edge, - at least one guide member fixed to the first crosspiece of the seat frame by fixing screws received in bores provided on the first crosspiece, the guide member being elongated and being configured to cooperate with the stiffening member, the guide member comprising a longitudinal housing shaped to at least partially house the stiffening member, the first edge of the seat and at least a portion of the first crosspiece of the seat frame, the housing being arranged in an upper portion of the guide member relative to the direction of mounting of the guide member on the first crosspiece, the guide member being configured to press and block the first edge of the seat and the stiffening member against the first crosspiece, the guide member comprises a plurality of indexing lugs distributed over the length of the guide member, each indexing lug being projecting and extending in a direction substantially perpendicular to the longitudinal direction in which the guide member extends, each indexing lug being configured to be positioned opposite and to be inserted into a respective through hole provided on the first edge of the seat and in a respective bore provided on the first crosspiece. - a seat frame comprising at least a first crosspiece and a second crosspiece positioned at a distance from the other and delimiting a seat space, - a seat made of stretchable material and shaped to at least partially cover the seat space delimited by the crosspieces (61, 62) of the seat frame, the seat comprising at least a first edge and a second opposite edge, the first edge being positioned on the first crosspiece of the seat frame and the second edge being positioned on the second crosspiece of the seat frame, - at least one stiffening member arranged on the first edge, the stiffening member extending at least partially along the first edge, - at least one guide member fixed to the first crosspiece of the seat frame by fixing screws received in bores provided on the first crosspiece, the guide member being elongated and being configured to cooperate with the stiffening member, the guide member comprising a longitudinal housing shaped to at least partially house the stiffening member, the first edge of the seat and at least a portion of the first crosspiece of the seat frame, the housing being arranged in an upper portion of the guide member relative to the direction of mounting of the guide member on the first crosspiece, the guide member being configured to press and block the first edge of the seat and the stiffening member against the first crosspiece,