Lumber is cut and planed to a standardized width and depth, which is specified in inches. Sizes such as the 2X4, or the 2X6 are well known typical sizes of often-used lumber.
The actual dimension, or size of a board differs from the nominal size, varying due to planing and shrinkage as the board is dried. This causes the actual dimension of the board to be slightly smaller than the nominal dimension or size.
Inversely, when wood is planed when it is green, the nominal size will be made slightly larger, sometimes as much as 1/16th of an inch bigger, to compensate for the shrinkage when green wood dries.
Chart for Average Actual verses Nominal Hardwood Lumber Sizes
The three numbers represent the following categories: Nominal Size, Actual Surfaced One Side, and Actual Surfaced Two sides:
1⁄2 in 3⁄8 in 5⁄16 in
5⁄8 in 1⁄2 in 7⁄16 in
3⁄4 in 5⁄8 in 9⁄16 in
1 in or 4⁄4 in 7⁄8 in 13⁄16 in
1+1⁄4 in or 5⁄4 in 1+1⁄8 in +1⁄16 in
1+1⁄2 in or 6⁄4 in 1+3⁄8 in 1+5⁄16 in
2 in or 8⁄4 in 1+13⁄16 in 1+3⁄4 in
3 in or 12⁄4 in 2+13⁄16 in 2+3⁄4 in
4 in or 16⁄4 in 3+13⁄26 in 3+3⁄4 in
Keep this in mind when you measure for projects, considering your standard "2x4" is actually 1+3⁄4 in by 3+3⁄4 in.