Clocks may be surface, flush or double dial mounted and are available in 10", 12", 15" or 18" round and 12" square. Flush mount clocks require a flush backbox. The minute impulse secondary clock receives an electric impulse from the master clock once each minute advancing it one minute. Each hour, rapid impulses are emitted from the master clock to the secondary clock system by means of a third wire, or reverse polarity on 2-wire versions, to provide hourly supervision of all secondary clocks. The hourly correction function will adjust secondary clocks, which are up to 20 minutes slow or up to 10 minutes fast.