Base rnd, each side: Multiple of 2 + corner sts.
Begin 1 st to left of corner st.
Rnd 1: Ch 3 (counts as dc), dc in each st around, placing 5 dc in each corner st, join with slip st to top of ch-3.
Rnd 2: Ch 2, BPdc around ch-3 and next dc together, *ch 1, [BPdc around next 2 dc together, ch 1]** to corner st, (BPdc, ch 1, BPdc) around corner st, rep from * around, ending last rep at **, join with slip st to first BPdc.
Rnd 3: Ch 1, sc in same st, *ch 3, sc in next dc; rep from * around, join with slip st to first sc. Fasten off.
If you are working this border in more than one color, join the second color in the top of the chain-3 behind the slip-stitch join of Round 1 so that the first-color framing is not interrupted.