Alt Code - History and Description

History and Description

Originally in MS-DOS, the user could hold down the Alt key and type a decimal number on the keypad. The BIOS would turn this directly into the character code of the same value, which (if the program did not interpret as a command) would place that same code on the screen, thus displaying that character from the current code page. For systems using English, this is code page 437. For most other systems using the Latin alphabet, this is code page 850. For a complete list, see code page.

These numbers became so well known and memorized by computer users that Microsoft was forced to preserve them when it transitioned Windows to use the Windows-1252 and similar international sets. Holding Alt and typing three digits (first one non-zero) would attempt to translate the code from CP437 to the matching glyph in the Windows code page. A leading 0 (zero) and then a number would produce the character from the Windows code page.

For instance, the combination Alt+161 yields "í" (Latin letter i with acute accent) which is at 161 in CP437 and CP850. Alt+0161 yields the character "¡" (inverted exclamation) which is at 161 in Windows-1252.

When Windows transitioned to Unicode the same thing had happened: the 0-leading codes had become so well known that a third method needed to be invented to produce Unicode code points. Although CP1252 is quite close to the start of Unicode and it would seem logical to just allow larger numbers to be typed, other international sets did not match. In addition some users were accustomed to the composition exiting after the third digit without releasing Alt, and numbers larger than 255 being translated modulus 256. To enable the third method, a user must set or create the registry key HKCU\Control Panel\Input Method\EnableHexNumpad with type REG_SZ to value 1 and reboot (logging out and logging back in is sufficient). Once the registry key is set, the following method can be used to enter Unicode codepoints:

• Keep the Alt key pressed. Press the "+" key on the numeric keypad.
• With the Alt key still pressed, type the hexadecimal number using the numeric keypad for digits 0-9 and the normal keys for a-f.

For example, Alt++11b will produce ě (e with caron).

The transition to Unicode actually made the older legacy Alt codes (the ones with no leading zero) more reliable, as all the glyphs in the legacy code pages have matching glyphs in Unicode, so they all work.