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An Alt code is a technique that is utilized on computers running either the Microsoft Windows operating system or disk operating system (DOS). The technique allows symbols and characters that are not present on the keyboard to be inputted through holding down the alternative character, or "Alt," key — of which there are usually two on a keyboard positioned on either side of the space bar — while typing in a code. The technique allows for localized keyboard layouts to produce language-specific characters and system symbols without requiring any hardware modifications.
To be able to utilize an Alt code, the number lock, usually designated as "Num Lock," on the keyboard must be active. By holding down the "Alt" key while simultaneously typing in a numerical code, Alt codes allow the user to display characters and symbols that are not usually available directly from a standard localized keyboard. The code needs to be entered using the numerical pad to the right of the main keyboard section of a standard keyboard.
This feature can be particularly useful when typing words in another language, to reproduce copyright and trademark symbols, and for entering system symbols into a text document. For example, holding down the "Alt" key and typing the numbers "0," "2," "3," and "3" will produce a lowercase e with an acute accent — é. The uppercase version of the letter can be achieved by pressing the "Alt" key, then "0," "2," "0," and "1." There are several websites on the Internet specifically listing all variations of Alt code and the resulting character display.
Some keyboards, such as laptop keyboards and smaller wireless keyboards, do not feature a separate numerical pad as standard. Alt codes can still be used on these keyboards by activating a built-in numerical keypad. The majority of smaller keyboards and laptop keyboards have alphanumeric key assignations whereby letter keys can be made to act as a numerical pad. By holding down the function key — usually designated as "Fn" — and the number lock key simultaneously, it is possible to activate the numerical keypad functionality. The alphanumeric keypad can then be used to enter an Alt code in the same way as with a standard keypad.
By typing in an Alt code, the user is actually identifying a location in a character set and bringing it to the screen. Codes that begin with a zero reference the Windows code page, which is often called Windows-1252. If the Alt code used does not begin with a zero, then it is making reference to a DOS-based code page.