Sewing the curve
Work in rows from the top of the quilt down. Each shell's round top fits into the lower curves of the two shells above it: match its side notches to their center notches and the top notch to the point where they meet. Pin the ends, sew with the concave piece on top, and press the curve toward the lower row.
How the template is drawn
Clamshells are circles of the finished width, touching in a row, with each row half a width above the last and shifted half a width, the spacing Stitched in Color uses for appliqué clamshells. What shows of each circle is a half circle on top and two quarter circles below that meet in a point, so a clamshell is as tall as it is wide. The free Love Patchwork & Quilting card template (Jo Avery, 61 mm) has this shape.
Example: 6 in clamshells with 1/4 in seams are cut from 6 1/2 in squares, 6 to a 40 in strip, so 40 shells take 7 strips (1 1/2 yd with the extra 1/8 yd).
Questions
How many clamshells do I need for a quilt?
Rows are half a shell apart. For a 60 by 72 in quilt of 6 in shells, that is about 11 across (one extra for the staggered ends) by 25 rows, or 275 shells, trimmed at the edges.
Can I appliqué clamshells with this template?
Yes. The allowance on the lower curves sits under the next row, so the same template works for piecing and for turned-edge appliqué.
Why is the bottom point rounded on the cutting line?
The finished point is sharp; the cutting line keeps a full seam allowance all round it, which makes a small curve below the point.
What is the smallest clamshell I can make?
The curves need room for the seam allowance: at 1/4 in seams the tool allows shells down to 1 1/2 in, but under 2 in, appliqué is easier than piecing.
How do I know the printout is the right size?
Measure the square at the top right of each page: it must be exactly 1 in (or 25 mm). If not, print again at 100% or Actual size, not Fit to page. From a phone, save a PDF and print that at 100%.
Sources: Stitched in Color, Easy appliqué clamshells from circles; Simply Quilting, clamshell cushion with free templates (Jo Avery).