Tutorial 18 - The Trapezium Bone
 
The trapezium bone is the "saddle seat" of the first metacarpal (the part of the thumb that's inside the hand). It's a bone partially responsible for the thumb's opposable motion.

We can see or feel it, if we rotate our thumbs in a circular motion. 

Study some xray images of hands while you do this. Look back at the joints on your own hands. Draw what you learn until it's second nature.