The APCMAMPS57 circular metal 57mm ball to 4-Hole AMPS adapter is compatible with 57mm (2.25 inch) mounts and pedestals, including those from Arkon or RAM Mounts Size D. (Mounts, holders, and devices sold separately.) The adapter's metal construction provides a sturdy connection for drilling into dashboards or other solid, flat surfaces, ideal for commercial or fleet use. The round design is a great alternative for locations and devices where a square adapter won't work.