LOCK is a function that locks part of a keyboard's keys into a distinct mode of operation, depending on the lock settings selected.
Description.
Most keyboards have three different types of "lock" functions:
Some laptops and compact keyboards also have a Function Lock - "FN" Lock. On these devices, a "Fn" modifier key is used to combine keys to save room and add non-standard functionality; a common use is merging the row with keys F1- F12 with keys that adjust settings such as display brightness, media volume and playback, and keyboard illumination. "Fn" Lock toggles the default output of these keys. 
Location.
The lock keys are scattered around the keyboard. Most styles of keyboards have three LEDs indicating which locks are enabled, in the upper right corner above the numpad. Some ergonomic keyboards instead place the lock indicators in between the key split. Some brands of keyboards have a "function mode" key (also called "F mode" or "Office Lock"), and may replace the scroll lock indicator with an office lock indicator. Office Lock, when enabled, will enable alternate functions of the function keys, meant for use with various word processing or email programs.
References.
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