The Acromioclavicular ligament is part of the acromioclavicular joint

Acromioclavicular joint

The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion and the clavicle. -Function:...

. It is divided into two parts: superior and inferior.

Superior Acromioclavicular Ligament

This ligament is a quadrilateral band, covering the superior part of the articulation, and extending between the upper part of the lateral 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 the adjoining part of the upper surface of the acromion.

It is composed of parallel fibers, which interlace with the aponeuroses of the Trapezius and Deltoideus; below, it is in contact 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....

 when this is present.

This ligament provides horizontal stability to the Acromioclavicular joint

Inferior Acromioclavicular Ligament

This ligament is somewhat thinner than the preceding; it covers the under part of the articulation, and is attached to the adjoining surfaces of the two bones.

It is in relation, above, in rare cases with the articular disk; below, with the tendon of the Supraspinatus.