Vanishing Line

Vanishing Line. Look at the 2 pieces of wood and you will clearly see 13 horizontal lines. Yet flip the frame upside down and there are only 12 lines!  How can this be? Some call this the "Sobriety Test", yet there is an interesting math basis for the puzzle. The idea for this wonderful illusion came from one of Martin Gardner's recreational math books.  Good for messing with friends, or a coffee table/bar- Vanishing Line.