MoriSorry for the late response. Don't have a ruler on hand, went to http://iruler.net/, which supposedly calibrates a correct 12-inch ruler by analyzing the monitor I am using (doesn't explain how!), and so when I set my 12-inch rest parallel and under the ruler, both the left and right sides matched up perfectly visually. I would guess that the millwork machinery would have been adjusted carefully by the miller to be 12 inches.