The discrepancy between the nominal dimensions and the finished dimensions of timber, such as a "2x4" stems from historical practices in the timber industry and the natural characteristics of wood. To understand this, we need to break down the process of harvesting and milling timber.

1. Harvesting Timber: Timber, or trees suitable for timber, are harvested from forests. Trees are typically cut into logs, which are then transported to sawmills for processing.
2. Sawmilling Process: At the sawmill, logs are cut into rough pieces of timber. Initially, the saw cuts the logs into larger sections, which are known as "flitches" or "cants."
3. Drying and Planing: After the initial cutting, the timber undergoes a drying process to reduce moisture content. This is important because wet or green wood is more prone to warping, shrinking, and splitting. Once dried, the timber is planed to achieve a smooth surface and consistent thickness.
4. Dimensional Changes: During the drying and planing processes, the wood undergoes dimensional changes. Wood shrinks as it loses moisture, and planing removes material from the surface, reducing its thickness and width.
5. Nominal vs. Actual Dimensions: The term "nominal dimension" refers to the size designation of the timber before it undergoes drying and planing. In the case of a "2x4," the nominal dimensions suggest a piece of timber that is 2 inches thick by 4 inches wide.

   However, the actual dimensions of the timber are smaller due to the dimensional changes during processing. After drying and planing, a "2x4" typically has actual dimensions closer to 1.5 inches thick by 3.5 inches wide.

6. Economics and Efficiency: The discrepancy between nominal and actual dimensions has economic implications for the timber industry. Historically, timber was cut to nominal dimensions that reflected the size of the wood when green or unprocessed. This allowed for standardized sizing and ease of reference in construction. However, as the timber industry evolved, there was a need to improve efficiency and reduce waste. By utilizing drying and planing processes, sawmills can produce smoother, dimensionally consistent timber while maximizing the yield from each log. This efficiency translates into cost savings for both producers and consumers.
7. Standardization and Industry Practices: Despite the difference between nominal and actual dimensions, the use of standardized sizing remains prevalent in the timber industry. This is because it facilitates communication, planning, and construction. Builders and contractors are familiar with nominal sizes and can work with them effectively, even though they understand that the actual dimensions will be slightly smaller.

In summary, the discrepancy between nominal and actual dimensions of timber like a "2x4" is a result of historical practices, the natural characteristics of wood, and the need for efficiency in the timber industry. Despite these differences, standardized sizing persists due to its utility in construction and familiarity among industry professionals.