The fibula is the thinner of the two long bones in your lower leg, the part below the knee. It sits on the outside. The thicker bone on the inside is the tibia — that is the one you can feel as your shin. You need the fibula, though actually it does not take as much of your weight as the tibia does; it helps keep the ankle steady and gives muscles a place to hold. Doctors say “a broken fibula” after a bad twist or a hard knock. It is a medical word. Ordinary people more often say they broke a bone in the lower leg, until they hear the doctor use the name. The pair of bones was thought to look like an old clasp, and the fibula was the pin.