A standard lumber trade designation indicating that a board has been factory-milled smooth on all four surfaces: both wide faces (surfaced) and both long edges (rip-cut and edge-jointed).

Other Designations

S1S (Surfaced 1 Side): Planed smooth on one face only; used when a uniform thickness is needed on one side while keeping the other rough.

S2S (Surfaced 2 Sides): Planed smooth on both wide faces to a consistent thickness, but the edges remain rough and un-squared.

S3S (Surfaced 3 Sides): Planed smooth on both faces, with one edge freshly jointed straight and square ($90^\circ$).