Cast on 136 sts. Work in K1, p1 Rib until piece measures 2 in. (5 cm) from beg.
Work in St st (k on RS, p on WS) until piece measures 36 in. (91.5 cm) from beg.
Work in K1, p1 Rib until piece measures 38 in. (96.5 cm) from beg. Bind off.
Fold piece in half, bringing cast-on edge and bound-off edges together, to measure 34 x 19 in. (85.5 x 48.5 cm). Place markers on each of the 19 in. (48.5 cm) sides, 7 in. (18 cm) below fold for armholes. Seam sides from marker down to lower edge. Weave in ends.