This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Alt code" – news ·newspapers ·books ·scholar ·JSTOR(March 2022) (Learn how and when to remove this message)

Onpersonal computers with numeric keypads that useMicrosoftoperating systems, such asWindows, manycharacters that do not have a dedicated key combination on thekeyboard may nevertheless be entered using theAlt code (theAlt numpad input method). This is done by pressing and holding theAlt key, then typing a number on the keyboard'snumeric keypad that identifies the character and then releasingAlt.^[1]

OnIBM PC compatiblepersonal computers from the 1980s, theBIOS allowed the user to hold down theAlt key and type a decimal number on the keypad. It would place the corresponding code into the keyboard buffer so that it would look (almost) as if the code had been entered by a single keystroke. Applications reading keystrokes from the BIOS would behave according to what action they associate with that code. Some would interpret the code as a command, but often it would be interpreted as an 8-bit character from the currentcode page that was inserted into the text the user was typing.^[2] On the originalIBM PC the code page wasCP437.

Some Eastern European, Arabic and Asian computers used other hardwarecode pages, and MS-DOS was able to switch between them at runtime with commands likeKEYB,CHCP orMODE. This causes the Alt combinations to produce different characters (as well as changing the display of any previously-entered text in the same manner). A common choice in locales using variants of the Latin alphabet wasCP850, which provided more Latin character variants. (There were, however, many more code pages; for a more complete list, seecode page).

PC keyboards designed for non-English use included other methods of inserting these characters, such as nationalkeyboard layouts, theAltGr key ordead keys, but the Alt key was the only method of inserting some characters, and the only method that was the same on all machines, so it remained very popular.^{[where?][clarification needed]} This input method is emulated by many pieces of software (such as later versions of MS-DOS and Windows) that do not use the BIOS keyboard decoding.

In the ASCII standard, the numbers 0-31 and 127 are assigned tocontrol characters, for instance,code point 7 is typed byCtrl+G. While some (most?) applications would insert abullet character• (code point 7 oncode page 437), some would treat this identical toCtrl+G which often was a command for the program.^{[citation needed]}

The Alt codes had become so well known and memorized by users that Microsoft decided to preserve them inMicrosoft Windows, even though the OS features a newer and different set of code pages such asCP1252. Windows includes the following processing algorithm for Alt code, which supports both methods:

• The familiarAlt+### combination retains the oldMS-DOS behavior, i.e., generates characters from the legacy code pages now called "OEM code pages".^[3] For instanceAlt+163 producesU+00FA úLATIN SMALL LETTER U WITH ACUTE. code point 163 inCP437 orCP850. The "OEM code page" selection served little or no other purpose other than to control what Alt codes produced, if other code pages were selected then these numbers could produce different characters, but it did not change the display. It was impossible to type characters that were not in the Windows code page, such as box-drawing symbols.

• The newAlt+0### combination (which prefixes a zero to each Alt code), produces characters from the newer "ANSI code pages".^[a][3] For instanceAlt+0163 results inU+00A3 £POUND SIGN (whenCP1252 is active, because it occupies position 163 in that code page). The active Windows code page could be different, which could change the effect of these Alt codes.

Later versions of Windows and applications such as Microsoft Word supported Unicode. As Unicode included all the characters in all the MSDOS code pages, this had the immediate benefit that all the old MSDOS Alt combinations worked, not just the ones that existed in the Windows Code Page. And far more software stopped changing how documents displayed or printed when the selected code page was changed.

In the IBM PC Bios typing an Alt code greater than 255 produced the same as that numbermodulo 256.^[2][4] Some applications retained this behavior, while others (in particular applications using the WindowsRichEdit control, such asWordPad andPSPad) made numbers from 256 to 65,535 produce the corresponding Unicode character.^[5] For instance,Alt+9731 in WordPad produces theU+2603 ☃SNOWMAN. If the Windows Code Page was set to CP1252 then all Unicode BMP characters exceptcontrol characters could be typed this way.

Because most Unicode documentation and character tables show the code points inhex, not decimal, a variation of Alt codes was developed to allow the typing of numbers in hex (using the main keyboard forA–F). To enable it, a user must set or create a string type (REG_SZ) value calledEnableHexNumpad in theregistry keyHKEY_CURRENT_USER\Control Panel\Input Method, assign the value data1 to it, and then reboot or log out/in. A leading+ then indicates hex input, for exampleAlt++11B will produceě (e withcaron).

There are manyother ways of typing arbitrary Unicode characters, such as theCharacter Map utility.

The Alt key method does not work onChromeOS,macOS,Linux or otheroperating systems and there is no evidence of interest in replicating it. However, numeric entry of Unicode characters is possible in mostUnix orUnix-like OSs by pressing and releasingCtrl+⇧ Shift+U, and typing the hex number followed by the space bar or enter key. For example,

• For theregistered trademark symbol®, typeCtrl+⇧ Shift+U,AE,↵ Enter.
• For theno entry sign⛔, typeCtrl+⇧ Shift+U,26D4,↵ Enter.

IfNum lock is disabled, attempting an Alt code may cause unexpected results in some applications, due to the controls used on the same key. For example,Alt+4 can be taken asAlt+←, causing a web browser to go back one page.

• Combining character
• Compose key for other operating systems
• Keyboard layout
• List of Unicode characters
• Numeric character reference
• Unicode input

• Character encoding
• Computer keyboards
• Input methods

• Articles with short description
• Short description is different from Wikidata
• Articles needing additional references from March 2022
• All articles needing additional references
• Vague or ambiguous geographic scope from October 2020
• Wikipedia articles needing clarification from October 2020
• All articles with unsourced statements
• Articles with unsourced statements from October 2020