Round Plate w/Ball. How it works: First choose a device adaptor. RAM offers a wide selection of adaptors and cradles for your specific device. Secondly choose an arm. Select an arm based on your desired length and placement. Lastly, choose a base. Select from a variety of bases to help secure your device where you need it. Ball Size Selection Guide: "B" Size balls are 1" diameter, hold up to 4 lbs., and are ideal for phones, cameras and cup holders. "C" Size balls are 1.5" diameter, hold up to 8 lbs, and are ideal for tablets and small monitors."D" Size balls are 2.25" diameter, hold up to 12 lbs., and are ideal for display integrations.