Why hertz in some countries is 50 Hz and in other countries is 60 Hz?

The standard voltage and frequency of alternating current (AC) electricity used in homes varies from country to country throughout the world. Typically, either-volt AC (V) or-volt AC (V) voltages are used. Also, most countries useHz ( Hertz or cycles per second) as the AC frequency. Only a handful useHz. The standard in the United States isV andHz AC electricity. However, due to fluctuations, the average measured voltage is VAC.