Dressed Lumber
Lumber that has been surfaced (planed) on one or more sides to achieve a smooth finish and uniform thickness, as opposed to rough-sawn lumber.
In-Depth Explanation
Dressed lumber is designated by how many surfaces have been planed: S1S (surfaced one side), S2S (surfaced two sides), S3S, or S4S (surfaced four sides, the most common). The dressing process removes saw marks and brings the board to a consistent, predictable thickness. S4S lumber is ready to use for most projects without further milling, making it convenient for DIYers and finish carpentry. However, the planing process reduces the board's dimensions from its rough size, which is why dressed lumber is thinner and narrower than its nominal designation. Dressed lumber commands a higher price than rough-sawn stock due to the additional processing.
Usage Example
We ordered S4S dressed lumber for the built-in cabinets so we could skip the milling step and go straight to cutting and assembly.