In computing, plain text is the contents of an ordinary sequential file readable as textual material without much processing, usually opposed to formatted text.
The encoding has traditionally been either ASCII, one of its many derivatives such as ISO/IEC 646 etc., or sometimes EBCDIC. Unicode-based encodings such as UTF-8 and UTF-16 are gradually replacing the older ASCII derivatives limited to 7 or 8 bit codes.
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The purpose of using plain text today is primarily a "lowest common denominator" independence from programs that require their very own special encoding or formatting (with due sacrifices and limitations). Plain text files can be opened, read, and edited with most text editors. Examples include Notepad (Windows), edit (DOS), ed, emacs, vi, vim, Gedit or nano (Unix, Linux), SimpleText (Mac OS), or TextEdit (Mac OS X). Other computer programs are also capable of reading and importing plain text. It can also be used by simple computer tools such as line printing text commands like type (DOS and Windows) and cat (Unix), but also for more complex activities like web browsers, i.e. Lynx and the Line Mode Browser.
Plain text files are almost universal in programming; a source code file containing instructions in a programming language is almost always a plain text file. Plain text is also commonly used for configuration files, which are read for saved settings at the startup of a program.
Plain text is the original and ever popular method of conveying e-mail. HTML formatted e-mail messages often include an automatically generated plain text copy as well, for compatibility reasons.
Text was once commonly encoded in ASCII, using 8 bits for one letter or other character, encoding 7 bits, allowing 128 values, and using the 8th as a checksum bit when transferring a file. This just allowed the ordinary Latin alphabet, transfer control codes, parentheses and interpunction, which annoyed computer users, especially Portuguese and Swedish users.
When data transfer became more stable, the 8th bit stopped being used as a checksum and was used to extend the character set by another 128 characters; these non-standard characters were encoded differently in different countries, and in a way that made multilingual texts impossible to encode. For instance, a browser may display ¬A rather than ` if it tries to interpret one character set as another.
At last Unicode was defined, which currently allows for 1,114,112 code values used for any modern text writing system and a lot of extinct ones, and is universal. For example, Unicode encodes characters for Chinese, Hebrew, and Cyrillic scripts as well as Latin. Some of these text formats may be quite complicated to process correctly, but they still contain no structural data, such as bold start and end markers, and are therefore plain text.
The ASCII codes before SPACE (= 32 = 20H) are not intended as displayable characters, but instead as control characters. They are used for diverse interpreted meanings. For example, the code NULL (= 0, sometimes denoted Ctrl-@) is used as string end markers in the programming language C and successors. Most troublesome of these are the codes LF (= LINE FEED = 10 = 0AH) and CR (= CARRIAGE RETURN = 13 = 0DH). Windows and OS/2 require the sequence CR,LF to represent a newline, while Unix and relatives use just the LF, and Classic Mac OS (but not Mac OS X) uses just the code CR. This was once a slight problem when transferring files between Windows and Unix systems, but today most computer programs treat this seamlessly.
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