The history behind the world's electrical frequency split
In the 1880s, the "War of Currents" pitted Thomas Edison's direct current (DC) systems against the alternating current (AC) systems backed by George Westinghouse and Nikola Tesla. AC won, mainly because transformers made it practical to send power over long distances at high voltage and then step it down for homes and businesses — something DC of that era couldn't do efficiently. But winning the AC-vs-DC argument didn't settle a second question nobody had answered yet: how many times per second should the current alternate?
Early AC systems in the United States used a scattered mix of frequencies — 25 Hz, 40 Hz, 50 Hz, 60 Hz, even 125 Hz and 133 Hz showed up in different cities, largely dictated by whatever generating equipment a given utility had already installed. The famous Adams Power Station at Niagara Falls, one of the first large-scale AC hydroelectric plants, ran at 25 Hz — a frequency that lingered in parts of the northeastern US and Ontario for industrial motors well into the 20th century simply because replacing that equipment wasn't worth the cost.
Westinghouse eventually settled on 60 Hz for most of its US systems. Part of the reasoning was mechanical: 60 Hz paired well with the rotational speeds of the steam turbines and generators being built in the US at the time, and higher frequencies allowed for smaller, cheaper transformers than 25 Hz had used, without the flicker problems that very high frequencies caused in early arc lighting.
Germany's AEG (Allgemeine Elektricitäts-Gesellschaft) — one of the dominant electrical equipment manufacturers of the era — built its early generators around 50 Hz, a round number that fit naturally with the metric-influenced engineering conventions of continental Europe. As AEG and other German manufacturers exported generating equipment and expertise across Europe and into European colonies and trading partners, 50 Hz spread with it. By the time other regions electrified, they largely followed whichever supplier — American or German — had built their grid, which is why the map of 50 Hz vs. 60 Hz countries today roughly traces old trade and colonial relationships as much as it does any technical argument.
Nowhere shows this history more directly than Japan. In 1895, Tokyo Electric Light Company purchased generating equipment from AEG in Germany, wiring eastern Japan for 50 Hz. Around the same time, a utility in Osaka bought equipment from General Electric in the United States, wiring western Japan for 60 Hz. That single pair of purchasing decisions, made independently by two companies, is why Japan still runs a 50 Hz grid in the east (including Tokyo) and a 60 Hz grid in the west (including Osaka) — split roughly along the Fuji River — over a century later, connected only by a handful of frequency-converting substations.
By the time international standards bodies got involved in the 20th century, both frequencies were already deeply embedded — not just in power plants, but in every motor, clock, and transformer built to run on one standard or the other. Converting an entire national grid from one frequency to the other means replacing or rebuilding generation equipment, transmission gear, and eventually consumer appliances all at once. The cost of switching has simply never been smaller than the cost of leaving things as they are, so the International Electrotechnical Commission recognizes both 50 Hz and 60 Hz as standard rather than picking a winner. Today, roughly 175 countries run at 50 Hz and 45 run at 60 Hz — a split that's really a fossilized record of who bought equipment from whom, over 130 years ago.