Base rnd, each side: Multiple of 6 + 5 + corner sts.
Begin 6 sts to left of corner st.
Rnd 1: Ch 1, sc in same st, *ch 1, skip 2 sts, [dc, (ch 1, dc) 3 times] in next st — large shell made, ch 1, skip 2 sts, sc in next st; rep from * around, omitting last sc, join with slip st to first sc.
Rnd 2: Ch 5 (counts as dc and ch-2), *(dc, ch 1, dc, ch 1, dc) in center ch-space of next large shell — small shell made, ch 2**, [dc in next sc, ch 2, small shell in center ch-space of next large shell, ch 2] to corner, (dc, ch 3, dc) in corner sc, ch 2; rep from * around, ending last rep at **, join with slip st to 3rd ch of ch-5.
Rnd 3: Ch 1, sc in same st, *[ch 2, (sc, ch 3, sc) in next ch-1 space, ch 5, (sc, ch 3, sc) in next ch-1 space, ch 2**, sc in +5 next dc] to corner space, ch 3, (sc, ch 5, sc) in corner ch-3 space, ch 3, sc in next dc; rep from * around, ending last repeat at **, join with slip st to first sc. Fasten off.