Shown with a cast-on count of 21 stitches.  

• Row 1 (RS): K21.
• Row 2: P21.
• Row 3: K1, M1L, k9, M1L, k1, M1R, k9, M1R, k1 (25 sts).
• Row 4: P25.
• Row 5: K1, M1L, k11, M1R, k1, M1L, k11, M1R, k1 (29 sts).
• Row 6: P29.
• Row 7: K1, M1L, k13, M1R, k1, M1L, k13, M1R, k1 (33 sts).
• Row 8: P33.
• Row 9: K1, M1L, k15, M1R, k1, M1L, k15, M1R, k1 (37 sts).
• Row 10: P37.
• Row 11: K1, M1L, k17, M1R, k1, M1L, k17, M1R, k1 (41 sts).
• Row 12: P41.
• Row 13: K1, M1L, k19, M1R, k1, M1L, k19, M1R, k1 (45 sts).
• Row 14: P45.
• Row 15: K1, M1L, k21, M1R, k1, M1L, k21, M1R, k1 (49 sts).
• Row 16: P49.
• Row 17: K1, M1L, k23, M1R, k1, M1L, k23, M1L, k1 (53 sts).
• Row 18: P53.