There are many variations of shell (or “fan”) stitch, from basic closed shells to open lace. They can even be inverted to form circles or stacked and separated by an entirely different stitch. Whether they’re worked in half-double, double, or triple the common attribute is grouped stitches, usually separated on each side by a single crochet to maintain structure.
Here is a variation of the basic shell, that I call an “Airy Shell” because it’s not quite closed and not entirely open. I don’t know if this technique is unique to me as there is not much in crochet that hasn’t been done before, but I came up with this as a means of speeding up the work. The pattern replaces the 2nd and 4th of 5 dc with chain ones. I used size 4, worsted weight, Lions Brand “Vanna’s Choice” and a size I aluminum hook to create placemats which are approximately 18″ x 12.5″
Chain a multiple of 6, + 2
Row 1: sc in 2nd ch from hook * sk 2, (1dc-ch1-1dc-ch1-1dc) in next ch, sk 2, 1 sc in next ch. * Repeat to end.
Row 2: ch 3 (counts as 1st dc), turn, ch 1, dc in same sp, *sc in 2nd dc of next shell, (1dc-ch1-1dc-ch1-1dc) in next sc. * Repeat to end.
Row 3 & up: If previous row ends on shell or half-shell: ch 1, turn, sc in top of shell, (1dc-ch1-1dc-ch1-1dc) in next sc. If previous row ends on sc: ch 4 (counts as dc and ch1), turn, dc in same sp, then sc in 2nd dc of next shell. This should keep your edges straight.