Mains frequency (Europe and most of the world)

AEG in Berlin standardised on 50 Hz in the 1890s, reportedly because 50 fitted the metric preference for round decimal numbers better than 60, and because their generator designs suited it. The choice spread with German electrical exports across Europe, then to Asia, Africa and Australia. Japan ended up split down the middle (Tokyo bought AEG generators and runs at 50 Hz, Osaka bought General Electric and runs at 60) a divide that still forces frequency converters between the two halves of the country.

Fifty hertz gives slightly lower iron losses and slightly larger transformers than sixty, and synchronous machines turn at 3000, 1500 and 1000 rpm instead of the North American speeds. Continental European grids have historically been held tightly enough that mains-synchronised clocks stayed accurate over months, with operators deliberately correcting accumulated time error; a 2018 dispute in the Balkans let the error build to six minutes before it was resolved.