Ancillary services refer to functions that help grid operators maintain a reliable electricity system. Ancillary services maintain the proper flow and direction of electricity, address imbalances between supply and demand, and help the system recover after a power system event. Ancillary services can include: Synchronized regulation, which is a service that corrects for short-term changes in electrical imbalances that might affect the stability of the power system. Contingency reserves, which are used to respond to an unexpected failure or outage of a system component, such as a generator, transmission line, circuit breaker, switch or other electrical element. Black-start regulation, which supplies electricity for system restoration in the unlikely event that the entire grid loses power. Flexibility reserves, which is an emerging concept for addressing variability and uncertainty on timescales longer than contingency and regulating reserves.