On/Off level controller is the simplest form of temperature control device. The output from the device is either on or off, with no middle state. This will switch the output only when the temperature crosses the set point. This prevents the output from chattering or making fast, continual switches if the cycling above and below the set point occurs very rapidly. They are commonly used in simple applications such as thermostats in heating systems and domestic refrigerators. They are modified to include a dead band for the error signal to reduce the sensitivity to measurement noise. This is sometime referred to as two-position or bangs control. One special type of on-off control used for alarm is a limit controller. This uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached. They are inexpensive and easy to implement. The disadvantage of these controllers is the cycling of the controlled variable, which is unacceptable in a quality control problem and usually leads to excessive wear and tear on the final control element.