The RAM-201LU, standard double socket arm with locking knob, has a socket at both ends that accommodates 1.5" ball bases. A single spring inside the arm opens the end sockets, allowing you to pivot the ball and socket connection. This socket technology allows for almost infinite adjustment and perfect viewing angles. The locking knob, includes a brass 1/4"-20 thread this allowing you to disengage the screwing of the knob. Once you have set your mount in the location you need, you can lock it in place and feel safe that your mounted device will stay right where you left it.