Clocks may be surface, flush or double dial mounted and are available in 10", 12", 15" or 18" round and 12" square. Once each hour the master clock supervises the accuracy of the secondary clock system by issuing a 60 second signal that will correct secondary clocks up to 59 minutes and 55 seconds slow. Every twelve hours the master issues a series of supervisory signals to correct units up to 11 hours, 59 minutes slow. The reference time for the hourly correction is the 58th minute. Twelve-hour correction occurs between 5:00 and 6:00 twice daily. Flush mount clocks require a flush backbox. Each clock is supplied with a pair of Molex® connectors.