What's the difference between the round and rectangular metal plates?The area of the rectangular metal plate is larger than the round ones', and it can provide greater adsorption force. If your phone is relatively heavy, we recommend using a rectangular metal plate.Tips: Before installation, Clean the area with Alcohol Pad thoroughly and let it dry completely! Do not use the mount until 12 hours pass, 3M adhesive needs time to set so that it fits!Please fix the car holder on a plastic or sleek (glass, metal) surface and not on leather or porous surface. We recommend using this car holder for cell phone with a slim case.