Shown with a cast-on count of 21 stitches.  

• Row 1 (WS): P21.
• Row 2: K10, (k1, yo, k1) in next st, k10 (23 sts).
• Rows 3, 5, 7, 9, 11, 13, 15, 17, 19, and 21: Purl.
• Row 4: K11, (k1, yo, k1) in next st, k11 (25 sts).
• Row 6: K12, (k1, yo, k1) in next st, k12 (27 sts).
• Row 8: K13, (k1, yo, k1) in next st, k13 (29 sts).
• Rows 10, 12, 14, 16, 18, and 20: K2tog, k12, (k1, yo, k1) in next st, k12, ssk.