The Add-an-Eighth Ruler is specially designed with a ridge that automatically adds a 1/8" seam allowance and provides a straight edge for your rotary cutter. Use it in foundation piecing when the pieces of fabric you are sewing are less than 1/4" high, so you don't add extra bulk to the seam. When sewing spirals, it's essential for doing a "Next Step Trim" on small spirals.