With two strands of Color A held together, loosely chain 82.
Base Row (Note Use both loops of chain for shells of base row and for the half shells that begin and end each row.) Two tr into 4th ch from hook – counts as 3-tr half shell, *skip 2 ch, shell into next ch, skip 1 ch, sc into next ch. Repeat from * across, ending with 3 tr in last ch – half shell made. Fasten off A.
Row 2 Attach 2 strands Color B held together to top of chain at beg of Base Row. Ch 3, 2 tr in same st – counts as 3-tr half shell, *sc in ch-1 space in middle of shell, shell in back loop of sc. Repeat from * across, skipping last sc and ending with 3 tr in last tr. Fasten off B.
Row 3 Attach A to top of chain at beg of previous row. Ch 3, 2 tr in same st, *shell in back loop of first sc, sc in center ch-space of next shell. Repeat from * across, ending with shell in last sc, 3 tr in last tr – half shell made. Fasten off A.
Repeat Rows 2 and 3 until there are a total of 48 rows, changing colors for each row and starting at same edge each time. Final row will be Color A, do not fasten off.
Rnd 1 Continuing with A, ch 4 (counts as tr), turn, (2 tr, ch 2, 3 tr) in same st, 3 tr between half shell and first full shell in previous (top) row. *3 tr in ch-1 space of full shell, 3 tr in sc. Repeat from * across to corner. Work (3 tr, ch 2, 3 tr) in corner. Working along side, make 3 tr in base of each t-ch (where rows join). At next corner, work (3 tr, ch 2, 3 tr). Along bottom, work 3 tr between bottom of half shell and first full shell and 3 tr in base of each shell and sc across, ending 3 tr between last full shell and partial shell, (3 tr, ch 2, 3 tr) in corner. Work along final side, making 3 tr in base of each outside tr (where rows join). End with slip st to top of ch-4.
Rnd 2 Slip st in next 2 tr and first ch of ch-2 space. Ch 4, (3 tr, ch 2, 4 tr) in corner ch-2 space. *4 tr between each 3-tr group to corner, (4 tr, ch 2, 4 tr) in corner. Repeat from * around, ending with slip st in top of t-ch. Fasten off A.
Rnd 3 Attach B in any corner ch-2 space, ch 4, (4 tr, ch 2, 5 tr) in corner space, *5 tr between each 4-tr group to corner, (5 tr, ch 2, 5 tr) in corner. Repeat from * around, ending with slip st in top of beg ch-4. Fasten off B. Weave in ends.