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 6 inch long.
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.