I have 3 "Add-a. . ." (I say this because there's an add-a-quarter and add-an-eighth, plus two length sizes). If you are paper-piecing, it sure makes it easier than using an index card or an old credit card. The lady who came up with this special ruler, Carolyn McCormick, came to our little town (pop.1200, if that)to teach us a class (it helps that her friend was originally from our town.) She explained how she came up with this ruler and the tedious process of getting someone to manufacture it. She ran a good class, we all enjoyed her and learned a lot. That evening she had a trunk show. Her miniatures were amazing.
I would recommend her if you need a program and lesson.