Sawmill lumber is measured and sawn in quarters on an inch.  For example, 1 inch thick lumber is called 4/4 lumber (pronounced “four quarter” and means the lumber is four 1/4-inches thick.  5/4 would be five 1/4″ increments, or 1.25″, 6/4 would be 1.5″, etc.  The thickness is measured at the thinnest spot used to establish the grade of the board.
Since hardwoods are generally more valuable than softwoods, and in order to make sure that the board is never sawn too thin, a “Hardwood Quarter Scale” is used by the sawyer.  The scale automatically adds 1/8″ to the thickness of the hardwood board is being sawn.  So a 4/4 hardwood oak board is really sawn to 4/4+1/8 inch while a 4/4 softwood pine board is sawn to only 4/4.  One of he most common mistakes I see is that most sawyers, in order to save lumber, will saw hardwood using the softwood scale and end up sawing boards too thin too usable after they are dried and planed.