The teresis considered a muscle of the arm, as opposed to a muscle of the upper back. There are two components of the teres, major and minor.
It originates on the lateral aspect of the scapula and attaches to the humerus (upper arm bone). It is often referred to as 'the lats little helper'. It is a thick but somewhat flat muscle.
It is commonly mistaken as a rotator cuff muscle, which it is not. It it does not attach to the capsule of the shoulder joint.
The teres major functions to medially rotate and adduct (bring together) the humerus and assist the latissimus dorsi in helping to pull the scapula and humerus downwards and backwards when the arm is raised. It also helps to stabilize the humeral head in the shoulder joint cavity.
The teres is an often ignored muscles even though it has great capability given its size, shape and location. Oftentimes what looks like an engaged teres (pumped up) is really a layer of fatty tissue.