For newbie carpenters and DIY'ers, one of the more confusing aspects of working with lumber is the fact that a 2x4 board is strictly a nominal term and doesn't reflect the actual dimensions of the board.
Let's face it ... why wouldn't this confuse somebody?
Our friends at Prowood Lumber explain it like this: "The nominal measurements are a board's size before it has been planed smooth (surfaced) on all 4 sides. The actual measurements are the final size. If your project calls for precise measurements, be sure to check your lumber before purchasing."
*Note that lumberyards which sell boards in actual "true" dimensions will have the material specifically marked as such to avoid confusion with the more common nominal dimensions.
Image copyright Simpson Strong-Tie