Regia 2-Ply Reinforcement Thread The unsung hero of hand-knit socks. This fine wool-blend thread holds up with your sock yarn at the heels and toes — the spots that wear out first — so your socks last for years instead of months. Knit it together with your sock yarn wherever you want extra durability. A little goes a long way, and one bobbin reinforces multiple pairs. Pick a color that blends with your favorite sock yarn (or contrasts, if you want the heel to make a statement).