The Synchronous electric clock. Now, not very often used as a main timekeeper in a household having been replaced by Quartz based movements in recent decades. During the 30s, 40s, 50s and into the 60s electric synchronous clocks were ubiquitous. Their accuracy is maintained by the mains frequency which is kept extremely accurate by the national power generating companies. Over the long term these clocks are the most accurate.
Look at the 'Real Time Frequency Data'. The frequency of the AC mains controls the speed at which an electric motor runs and so maintains timekeeping via wheelwork to drive seconds, minutes and hour hands.