In this set, the L Size (9") Dual Knuckle Arm has a ball head with 1/4"-20 male thread on one end that is compatible with most cameras & camcorders. This arm is constructed by linking two S size arms with a 3" extension. The four ball heads of the arm allow users to adjust the mounted cameras to precise positions that suit their special needs. The extension arm has three 1/4"-20 and two 3/8"-16 female thread holes that allow for mounting devices using an additional Super Knuckle Arm. The Wheelchair Tube Clamp is equipped with a ball head that directly mounts into the Super Knuckle Arm. The Clamp can be tightly mounted onto wheelchair frame tubes and provides a very stable base. Given that there is no universal diameter for wheelchair tubes, three plastic spacers and a rubber strip are included allowing the clamp to fit 3/4" (19 mm), 7/8" (22.2 mm), 1" (25.4 mm), 1-1/8" (28.6 mm), 1-1/4"(32 mm) tubes. It is recommended to leave the tube clamp installed on the wheelchair so that it does not need to be remounted. The Super Knuckle Arm is easily released from the Wheelchair Tube Clamp by loosening the central knob of the arm. This set mounts a camera or camcorder within a flexible 9" (22.8 cm) radius of a wheelchair tube.