The thin bone beside the shin
Your lower leg has two bones side by side. The big one in front is the tibia. The thin one, on the outside of your leg, is the fibula.
The fibula is long and skinny. It sits deep under the muscles of your calf, so you can only feel its two ends.
Try it: feel the outside of your leg, just below your knee. That small, hard bump is the top of your fibula.
Here's a surprise: the fibula doesn't touch your thigh bone. It isn't part of your knee joint at all.
The outside ankle bump
Now feel the big bump on the outside of your ankle. That's the bottom end of your fibula!
Feel the bump on the inside of your ankle too. That one is your tibia. The two bumps hold the top of your ankle bone between them, like a hand gripping a ball.
Look closely: the outside bump sits lower than the inside one. Check yours!
Light work, big help
The fibula carries only a small part of your weight. The tibia does the heavy lifting.
So what does the fibula do? Many muscles of your lower leg and foot hold on to it. They help you point your toes, stand on tiptoe and keep your balance.
It also keeps your ankle steady, so it doesn't wobble so much when you walk on bumpy ground.
Named after a pin
Fibula is a Latin word for a pin, or a clasp. People long ago used clasps like this to hold their cloaks closed.
Some of those old clasps worked a lot like safety pins. People say the thin fibula beside the thick tibia looks like the pin part of a clasp.