Used to easily calculate the percentage to increase or decrease any stencil or paper pattern to fit into a block or border.  

This allows you to go to the copy machine and enter the correct percentage the first time without having to use the trial and error method.

For example, suppose you have a 5" stencil and you need it to be 7 inches to fit in the border or block. So you line up the 5 on the inner ring, with the 7 on the outer ring. Follow the arrow to the percentage window, and it will point to the exact percentage needed to enlarge a 5 inch design to a 7 inch design. In this case it would be 140%, so this is the value you would enter in the copier to enlarge it.  

The picture above shows this example.