Improved Clamshell is a high-density design perfect for traditional quilts. You will be surprised at how forgiving this design quilts, and it solves the problem of accidentally touching the shell below. My customers say this design looks almost hand-quilted because of the one-quarter inch echo look across the clamshell. This is my Mothers favorite design. (Skill: Advanced) 12 feet long, 11.5 inches, 4 Rows