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Encyclopedic overview
The Caps Lock key on a PC keyboard with US keyboard layout (near upper-left corner, below the Tab key and above the left Shift key)
Caps Lock ( ⇪ Caps Lock) is a button on a computer keyboard that causes all letters of bicameral scripts to be generated in capital letters. It is a toggle key: each press reverses the previous action. Some keyboards also implement a light to give visual feedback about whether it is on or off. Exactly what Caps Lock does depends on the keyboard hardware, the operating system, the device driver, and the keyboard layout. Usually, the effect is limited to letter keys. Letters of non-bicameral scripts (e.g. Arabic, Hebrew, Hindi) and non-letter characters are generated normally.
Excerpted from Wikipedia’s “caps lock” article, available under the CC BY-SA 4.0 licence.