Why is a 2×4 not a 2×4 you ask.  The world of dimensional lumber changed when quality became an expectation.  In years long past, when lumber mills cut dimensional lumber, they actually cut the trees to the specific size of 2” by 4” and 2” by 6”, etc.  The fact is, they still do.  So, what changed?  Quality changed.  Following the rough cutting process, the lumber is now sent through planers to “clean up” the lumber for sale to end-users.  This process has changed the size of the 2×4 to 1 ½” by 3 ½” and the 2×6 to 1 ½” by 5 ½”.  Other sizes of dimensional lumber have also changed to consistent sizes through this process.