S2S, S3S, S4S (Surfacing Standards)

S2S, S3S, and S4S describe how many faces of a board a mill has already surfaced (planed flat and smooth) before selling it: S2S is surfaced on its two broad faces only, leaving both edges rough-sawn; S3S adds one straightened edge; S4S -- surfaced four sides -- is flat, straight, and square on all four faces, ready to build with directly. Dimensional lumber framing stock is always sold S4S; hardwood is more often sold S2S or rough, since many woodworkers prefer to mill their own final dimensions from stock that's still slightly oversized.

Buying S2S instead of S4S is usually a cost tradeoff, not a quality one -- S2S is cheaper because the mill did less work, and it's the right choice specifically when a project's final dimensions aren't locked in yet and you'd rather rip and joint to your own exact sizes anyway.