This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The dislocations of the shoulder are to be studied from the anatomical and not from the clinical standpoint. A knowledge of the anatomical construction of the various parts involved is to be applied to the explanation and elucidation of the methods of production, the signs and symptoms observed, and the procedures necessary for reduction.
A posterior luxation is one in which the head goes posterior to the glenoid cavity and usually rests beneath the spinous process of the scapula, hence this is called subspinous dislocation.
When the head is luxated anteriorly it may pass so far inward as to rest between the coracoid process and the clavicle; hence this form is called subclavicular.
When the head does not pass so far inward, but rests on the anterior edge of the glenoid cavity below the coracoid process, it is called a subcoracoid luxation.
When it rests on the anterior and lower edge of the glenoid cavity, sometimes on the long head of the triceps muscle or just anterior to it, it is called a subglenoid luxation.