(cosmetic updates 5/10/23)
When instructions call for a 2×4 piece of lumber, “inches” is not needed because it is a nominal measurement. (The actual measurement, which depends on the dryness of the material and the milling methods, is about 1 5⁄8 inches by 3 5⁄8 inches.) The same goes for a 1x piece of lumber, which actually is only about 3⁄4 inch thick.
The nail size unit is called a “penny” and is abbreviated with the lowercase letter d. It indicates the length of the nail. For example, a 2d (2 penny) nail is 1 inch long. The length increases 1⁄4 inch for each penny after that. So, a 4d nail is 1 1⁄2 inches long. The measurement applies to common, box, casing, and finishing nails. Brads and small box nails are specified by their actual length and shank size (3⁄4-inch x 16 brad or 3⁄4-inch x 18 brad).
Standard order: Width x Height x Depth
Use Dimension X in measurements: