On personal computers running the Microsoft Windows or DOS operating systems, additional characters to those available in the current keyboard layout can be typed using an Alt code: pressing and holding the Alt key while entering a character code with the keyboard's numeric keypad.

Often this is the best or only method many computer users know for entering non-ASCII characters. Due to its popularity, the input method has been duplicated, including the use of obsolete code pages, to other operating systems. Many Wikipedia articles on various characters will include how to type that character using Alt codes for code page 437.

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.

ALT may refer to:

A code is a rule for converting a piece of information (for example, a letter, word, phrase, or gesture) into another form or representation (one sign into another sign), not necessarily of the same type.

In communications and information processing, encoding is the process by which information from a source is converted into symbols to be communicated. Decoding is the reverse process, converting these code symbols back into information understandable by a receiver.

One reason for coding is to enable communication in places where ordinary spoken or written language is difficult or impossible. For example, semaphore, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.

In information theory and computer science, a code is usually considered as an algorithm which uniquely represents symbols from some source alphabet, by encoded strings, which may be in some other target alphabet. An extension of the code for representing sequences of symbols over the source alphabet is obtained by concatenating the encoded strings.