Chain in multiples of 7 + 3 chains.
Row 1: (3dc, ch 2, 3dc) in the seventh ch from hook (shell made). *[skip the next 6 chs, work shell in next ch], Repeat from * to the last 3 chs, dc in last ch.
Row 2: Ch 3, turn. Work shell in each ch 2 sp across, dc in top of beginning ch.
Repeat row 2 for pattern.