Humerus is bone of upper arm. It has hemispherical head at it's proximal end. The distal end of humerus with a pulley like part or trochlea that articulates with ulna.
The proximal end of radius has disc like head that articulates with humerus bone. At the proximal end of ulna there is a prominent process called 'Olecranon process’ that forms elbow joint with humerus bone. The monoaxial hinge joint is present between them.