The 51.1 cm (20.1 inch) [434.4 x 271.9 mm] measurement refers to the size of the screen for a specific device that our website sells screen protectors for. This information is important for customers to ensure they purchase the correct size screen protector to fit their device. By providing the exact measurements, customers can be confident that they are getting a screen protector that will fit their device perfectly, providing maximum protection against scratches, cracks, and other damage. Our website offers a range of screen protectors specifically designed for this device, ensuring customers can find the perfect fit for their needs.