The “Shoulder” is the key part of the Fourth Arrow Camera Arm System. It is the joint between the base and the arm. The Fourth Arrow Shoulder allows you to affordably own a base for every treestand set. The shoulder slides firmly and locks into the base. After the shoulder is leveled with the included tool, the Arm may be placed in the socket of the shoulder and you are ready to video your hunt! The Fourth Arrow Stiff Arm Pro Shoulder is rated to a higher weight limit than the Stiff Arm and Carbon Arm Shoulder and comes with a Cam Lever to strengthen the Shoulder/Base connection.