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The term Alt codes is used to refer to a number of Unicode input methods that allow characters to be entered by typing a character's code point in concert with the Alt key.

• In Microsoft Windows, symbols and accented characters not found on the keyboard are inserted by holding down the Alt key and typing number(s) on the numeric keypad, see Windows Alt keycodes. Numbers that are used with the Alt key to generate a character, representing the Decimal numeric entity code; the WordPad editor can be used to generate characters from many Unicode blocks. Note, Internet Explorer does not support typing in decimal entity values more than 255. (This is immaterial, since Windows takes modulo 256 of Alt codes greater than 255 anyway; ref. Windows Alt keycodes.) Characters are stored on your computer as numbers, each letter or symbol is represented by a number. To make sure that text looks the same on all computers, there is a standard called Unicode that defines which numbers are mapped to which characters. Due to the number of alphabets in the world, Unicode defines tens of thousands of characters.

Note: doing certain alt-key sequences in certain web browsers can cause unexpected results (a blank page, enter, expired cache, etc).

• A variant of alt codes are Alt-X codes: they can be applied in WordPad and Microsoft Word 2002/2003 for Windows; the keystroke Alt + x will convert a Selected hexadecimal number to the corresponding character; see Unicode input methods, Windows Alt keycodes. Note, IE does not support this feature.
• The Linux console supports Alt codes using the same method as with Windows. In VIM editor, first type Ctrl-V u, then type in the hexadecimal number of the symbol or character that you want, and it will be converted into the symbol.
• In Mac OS X and in Mac OS 8.5 and later: choose the Unicode Hex Input keyboard layout. Hold down the Option key, and type the four-digit hex Unicode code point. Release the Option key; the equivalent character will appear. To type accents whilst using a standard Roman keyboard, hold down Option key then press `, e, i, n, and u for grave, acute, circumflex, tilde and umlaut respectively. You can also use Option in conjunction with other keys to produce various other characters.^[1]
• In GTK+ applications, press Ctrl-Shift-U and then type the Unicode number (in hexadecimal) with Ctrl and Shift held down. Later versions do not require Ctrl and Shift to be held down while typing the number. GTK+ versions before version 2.10 require that you hold down Ctrl and Shift while entering the number (Ctrl-Shift-U does not exist in those versions).

• A variety of codepoint-based input methods that do not use the term alt codes are provided by various programs and environments; see Unicode input methods for a full list. Cardbox Software has created a free utility that extends the alt codes system to non-microsoft applications.

• Entering characters with a utility like Quick Key (download) is much preferable to memorizing alt codes, although Quick Key also provides alt code reference. Proprietary alternatives include PopChar and FrKeys.

• For heavy usage that justifies relearning key positions, a new keyboard layout is recommended.

• Quick Key (download) (Open Source Keyboard Extension)
• Unicode Input Utility (Allows alt-codes on all programs)
• Mophy Accents (Simplifies typing accented characters)
• PopChar Demo Version (Similar to Quick Key)
• FrKeys

• Windows Alt keycodes
• Combining character

1. ^ typing special and accented characters

• Printable List of Alt Codes (UsefulShortcuts.Com)
• Windows Alt Key Codes (Penn State)
• Online Character Pickers (Richard Ishida, W3C)
• Keyboard Help - Learn how to create world language accent marks and other diacriticals on a computer