Extra buttons, switches and rotaries mounted on your cockpit.
Steering wheels only have so many buttons. A button box adds dedicated switches for ignition, lights, wipers, pit limiter and brake bias, which is especially useful in endurance and truck sims.
Most button boxes connect over USB and work on PC with any wheel base. You can add more than one to a build.
Prices run from around €40 for a basic panel of toggle switches up to €300+ for a unit with a rotary encoder and an OLED display. Mount position matters as much as the feature set: a button box bolted to a fixed cockpit rail stays put under hard braking, while one on a desk clamp can shift unless it's secured separately from the wheel mount.