RFID technology allows for securely holding 2 to 3 cards with room for a little cash. Measuring 3.63 by 2.50 inches, works with most phones and tablets without obstructing the camera. 3M adhesive backer. Adheres well to most flat surfaces. Not recommended for use on fabric or silicone. For devices with glass backs (iPhone X, 8/8 Plus, etc.) or extremely smooth surfaces, a case is needed for good adhesion.