The bones are parallel. The forearm appears wider and flatter.
Your own arm is the best 3D reference you have. anatomy for sculptors arm and hand in motion pdf free
Look for the "S-curves" that flow from the shoulder down through the fingertips. The bones are parallel
Think of the upper arm as a cylinder, the elbow as a hinge, and the palm as a flexible box. Conclusion the elbow as a hinge
The bony bump on the pinky side of the wrist. Note that this stays stationary while the radius (thumb side) rotates around it. 2. Pronation and Supination: The Great Twist