With the color key tops, typing is more fun! Please arrange the keyboard as you like. It is an optional product that supports (PD-KB400W, PD-KB400B, PD-KB400WN, PD-KB400BN, PD-KB420W, PD-KB420B, PD-KB400WS, PD-KB400WNS, PD-KB420WS) in HHKB Professional series. Please use them for exchanging key top ESC (blue) key and Control (red) key.