Clamshell quilting can be done with a ruler aid, or freehand. I find freehand only fun if the grid of the clamshell is 1” or smaller. Since one edge of my lavender lines ruler has a 1” half circle cutout just for this purpose, I used it in a recent quilt sample. I love it, since there isn’t any marking necessary beyond a base line. Stencils are also a great way to mark a clamshell for quilting. In all these cases, pausing and adding an embellishment at the corner of each repeat is a terrific way