This allows you to do circles without crossing your hands. Mark cross-lines for the center of your circle. Align the cross-lines of the guide with your drawn cross-lines and stitch down the concave side "innie side". Turn the ruler around and align your cross-lines of the guide again and stitch up the convex side "outie side". Small curves work well for designs inside blocks. With both the concave and convex curves the same, continuous curve can be done without crossing your hands.