The elbow is the joint between the upper and lower arm. Three bones make up the elbow joint: the upper arm bone (humerus) and the two bones in the forearm (radius and ulna). The joint between the humerus and the ulna is a hinge joint that allows the elbow to only bend and straighten (flex and extend). The joint between the humerus and the radius allows the forearm to rotate: it can turn inward so the palm is down (pronation) or outward so the palm is up (supination). Many muscles that move the wrist and hand attach around the elbow joint.