The 51.1 cm (20.1 Inch) [434.4 x 271.9 mm] measurement refers to the screen size of a specific device for which our website sells a range of screen protectors. This information is important for customers to ensure they purchase the correct size screen protector for their device. The dimensions provided in millimeters also allow for precise measurements and compatibility with various brands and models. By offering a range of screen protectors for this specific device, our website aims to provide customers with the best protection for their device's screen.