First off: The arm has four parts. These parts may vary depending on the position of the arm.
Second: There are two basic arm positions--Straight and Bent.
Third: Parts for Straight are The upper upper arm (top of your arm to the middle of your upper arm), the lower upper arm, than the upper forearm, then lower forearm.
Fourth: The Bent is upper upper arm, lower upper arm, elbow, and forearm. The distinction between upper and lower forearm is lost in this pose. Also, the elbow has three points, not one.
Rosie the Riveter is an excellent example of bent arm. Just look at little Rosie!
