The 51.1 cm (20.1 inch) [409.4 x 307.3 mm] measurement refers to the size of the screen that our website sells screen protectors for. This size is commonly found in various electronic devices such as computer monitors, laptops, and tablets. By providing screen protectors specifically designed for this size, our customers can ensure that their devices are protected from scratches, cracks, and other damages that can occur from daily use. Our range of screen protectors offers different materials and features to suit different needs and preferences, providing our customers with a variety of options to choose from.