A gusset, according to Merriam-Webster, is "a usually diamond-shaped or triangular insert in a seam (as of a sleeve, pocketbook, or shoe upper) to provide expansion or reinforcement." In this case, it provides shaping to fit the thumb and give it room to--well, to do what thumbs do!Beginning the Gusset
Rnd 1: K1, inc one st by working (p1, k1) both in next st, inc one st by working (k1, p1) both in next st, k to 1st marker, work Cable Stitch, k to end of rnd.
Rnd 2: k1, p1, k2, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 3: k1, inc one st by working (p1, k1) both in next st, k2, inc one st by working (k1, p1) both in next st, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 4: k1, p1, k4, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 5: k1, inc one st by working (p1, k1) both in next st, k4, inc one st by working (k1, p1) both in next st, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 6: k1, p1, k6, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 7: k1, inc one st by working (p1, k1) both in next st, k6, inc one st by working (k1, p1) both in next st, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 8: k1, p1, k8, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 9: k1, inc one st by working (p1, k1) both in next st, k8, inc one st by working (k1, p1) both in next st, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 10: k1, p1, k10, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 11: k1, inc one st by working (p1, k1) both in next st, k10, inc one st by working (k1, p1) both in next st, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 12: k1, p1, k12, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 13: k1, inc one st by working (p1, k1) both in next st, k12, inc one st by working (k1, p1) both in next st, k to 1st marker, work Cable Stitch, k to end
Rnd 14: k1, p1, k14, p1, k to 1st marker, p3, k8, p3, k to end of rnd.
Rnd 15: k1, inc one st by working (p1, k1) both in next st, k14, inc one st by working (k1, p1) both in next st, k to 1st marker, p3, k8, p3, k to end
Rnd 16: k1, p1, k16, p1, k to 1st marker, p3, k8, p3, k to end of rnd.