Before the computer era, the term referred to electrical clocks that are synchronized periodically by an electrical pulse through dedicated wiring issued by a master clock in the same building. From the late 19th to the mid 20th centuries, electrical master/slave clock systems were widely installed in public buildings and business offices, with all the clocks in the building synchronized through electric wires to a central master clock. These older styles of slave clocks either keep time by themselves, and are periodically corrected by the master clock, or require impulses from the master clock to advance.