This ruler has a lip at 1/4 inch from the edge. It helps you cut the fabric pieces in paper piecing at 1/4 inch from the sewing line, allowing you to position them precisely and reducing the amount of fabric in the seam allowances. This ruler is 12 inch long, and very useful for larger blocks.
You can find an extensive instruction of the paper piecing technique on the website of Maaike Bakker, where you can also find a lot of paper piecing patterns: www.paperpiecingpleasure.com. The directions are at the bottom of the page: Directions Paper Piecing. You will also find the use of the add-a-quarter ruler here. Or visit www.addaquarter.com for tutorials.