The 51.1 cm (20.1 Inch) [409.4 x 307.3 mm] is the screen size of a specific device that our website sells a range of screen protectors for. Knowing the exact screen size is important when selecting the appropriate screen protector to ensure a perfect fit and maximum protection. Our website offers a variety of screen protectors designed specifically for this device, including tempered glass, anti-glare, and privacy screen protectors. With our range of options, customers can choose the best screen protector to suit their needs and protect their device from scratches, cracks, and other damage.