The designation refers to a common piece of lumber used extensively in construction. The numbers represent the nominal dimensions of the wood in inches. These dimensions, however, are not the actual, final measurements of the board after it has been processed.
Understanding the discrepancy between the nominal and actual sizes requires knowledge of the lumber milling process. Initially, the rough-cut lumber does indeed measure two inches by four inches. However, during the drying and surfacing stages, the wood shrinks and is planed smooth, resulting in smaller, more consistent dimensions. This process provides a stable and workable product, essential for structural integrity in building projects. The practice of retaining the original nominal size in the name provides a historical link to the rough-cut origins and simplifies lumber identification and ordering. The size is universally known within the construction industry.