The “2x4” name comes from the size of the rough cut timber when it is sawed out of the tree trunk. It starts as two inches by four inches. However, by the time the board makes it to the lumber yard, it has been milled down to 1.5 inches by 3.5 inches in order to remove bark, gashes, and other imperfections. Same thing with “2x6” boards or larger.

Now, this rule really only applies to building timbers. Other means of measurement apply if you were to go looking for, say, hardwood boards.