They sound like planets but they are not! Your elbow joint is made up of three arm bones.
One of these bones is called the radius, which is the bone that runs from the elbow to the thumb side of your wrist.
The other two bones that make up your arm joint are called the ulna and humerus. The ulna is the bone that sits next to your radius and runs from your elbow to the little-finger side of the wrist. The humerus is your upper arm joint.
Did you know?
The length from your elbow to your wrist is the same as the length of your foot!