Baby Tube Socks #2 Ribbing

Ribbing: 4 knit stitches (through back loops), 4 purl stitches. Shift ribbing one stitch to the left every 4 rounds (so next: 1 purl stitch, 4 knit stitches, 3 purl  ... ) Continue until sock is 13 (1-3 months), 14 (6-9 months) or 15 (12-18 months)