Negative feedback is created when the output of a system is subtracted from its input (or command) to form the error signal. The error signal is normally fed into a controller, and the resulting signal is applied to the process to be controlled (the plant). Most controllers are either of the P (proportional), PI (proportional-integrator) or PID (proportional-integrator-derivator) kind. Once a negative feedback loop is in action in a servo system, it is said to be in closed loop mode, by opposition to the open loop mode, where the command is fed directly to the controller without any negative feedback loop.