This replacement for the footbar / ruler fits full-sized Das Keyboard 4 products, including the 4 Professional, 4 Professional for Mac, 4 Ultimate, 4 root, and 4Q. It does not fit the Das Keyboard 4C TKL. The footbar attaches magnetically to the bottom of the keyboard to raise the angle the keyboard, features improved rubber feet to prevent the keyboard from sliding on your desktop and provides a ruler that is always within reach (trust us, you'll thank us later).