The posterior sternoclavicular ligament is a band of fibers, covering the posterior surface of the sternoclavicular joint. It is attached above to the upper and back part of the sternal end of the clavicle

Clavicle

In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...

, and, passing obliquely downward and medialward, is fixed below to the back of the upper part of the manubrium sterni.

It is in relation, in front, with the articular disk

Articular disk

The articular disk is a thin, oval plate of fibrocartilage present in several joints which separates synovial cavities. This separation of the cavity space allows for separate movements to occur in each space....

 and synovial membranes; behind, with the Sternohyoideus and Sternothyreoideus.