RAM® Double Socket Arm Mirror Designed into the mount is

Designed into the mount is a 1" diameter patented rubber ball and socket system with adjustment points at the both ends of the socket arm

one round 2

While the base can support up to a maximum post width of 4"

The round base includes a 1" B size rubber ball and contains pre-drilled holes

RAM® Double Socket Arm Mirror Designed into the mount isThe RAM E 201U, double socket arm, has a socket at both ends that accommodates 3. 38" 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.