S4S (Surfaced Four Sides)
Lumber that has been planed smooth on all four faces. Standard retail dimensional lumber is S4S, which reduces the rough-sawn size to the smaller actual dimensions.
In Detail
S4S milling removes roughly 1/2" from each dimension — a rough-sawn 2" × 4" becomes a finished 1.5" × 3.5". Other surfacing options: S1S (one side), S2S (two sides), S1S2E (one side, two edges). Rough-sawn lumber is sold at full nominal dimensions and is specified when a rustic appearance or maximum dimension is needed.