The 51.1 cm (20.1 Inch) [409.4 x 307.3 mm] refers to the size of the screen that our website sells screen protectors for. This size is commonly found in a range of devices such as laptops, desktop monitors, and all-in-one computers. By providing screen protectors specifically designed for this size, our customers can ensure that their devices are protected from scratches, smudges, and other types of damage. Our range of screen protectors includes options such as tempered glass, anti-glare, and privacy filters, ensuring that our customers can find the perfect protection for their device.