What is the difference between nominal sizes and actual sizes?

Dimension lumber is identified by nominal sizes of rough lumber. Lumber that has been planed for a smooth surface has actual sizes that are less than the nominal sizes.

Nominal	Actual
2x4	1-1/2 x 3-1/2
2x6	1-1/2 x 5-1/2
2x8	1-1/2 x 7-1/4
2x10	1-1/2 x 9-1/4
2x12	1-1/2 x 11-1/4
4x4	3-1/2 x 3-1/2