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 computer monitors and other electronic devices, and our range of screen protectors is designed to fit this specific size perfectly. By using a screen protector, customers can protect their device's screen from scratches, smudges, and other types of damage, ensuring that their device stays in top condition for longer. Our website offers a variety of screen protectors for this size, including tempered glass, anti-glare, and privacy options, allowing customers to choose the protection that best suits their needs.