In computing, plain text is a term used for an ordinary "unformatted" sequential file readable as textual material without much processing. The encoding has traditionally been either ASCII, one of its many derivatives such as ISO/IEC 646 etc., or sometimes EBCDIC. No other encodings are used in plain text files which neither contain any (character-based) structural tags such as heading marks, nor any typographic markers like bold face, italics, etc. Unicode is today gradually replacing the older ASCII derivatives limited to 7 or 8 bit codes. It will probably serve much the same purposes, but this time permitting almost any human language as well as important punctuation and symbols such as mathematical relations (≠ ≤ ≥ ≈), multiplication (× •), etc, which are not included in the more restricted ASCII set

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, vi, vim or Gedit (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).

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. [edit] Related terms The related term, plaintext, is most commonly used in a cryptographic context, while cleartext usually refers to lack of protection from eavesdropping. Usage of these terms is such that there is some confusion amongst them, especially among those new to computers, cryptography, or data communications.

[edit] Character encodings Main article: character encoding 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 especially Portuguese and Swedish[citation needed] computer users. Therefore, when data transfer became more stable, the remaining 128 values were encoded, everywhere differently, and in a way that made multilingual texts impossible to encode. 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. For example Unicode codes Chinese, Hebrew, Cyrillic 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

Through the use of the multipart type, MIME allows messages to have parts arranged in a tree structure where the leaf nodes are any non-multipart content type and the non-leaf nodes are any of a variety of multipart types. This mechanism supports: simple text messages using text/plain (the default value for "Content-Type: ") text plus attachments (multipart/mixed with a text/plain part and other non-text parts). A MIME message including an attached file generally indicates the file's original name with the "Content-disposition:" header, so the type of file is indicated both by the MIME content-type and the (usually OS-specific) filename extension reply with original attached (multipart/mixed with a text/plain part and the original message as a message/rfc822 part) alternative content, such as a message sent in both plain text and another format such as HTML (multipart/alternative with the same content in text/plain and text/html forms) image, audio, video and application (for example, image/jpg, audio/mp3, video/mp4, and application/msword and so on) many other message constructs

[edit] Control codes Main article: newline The ASCII codes before SPACE (= 32 = 20H) are not intended as displayable characters, but instead as control characters. They are used for a diversity of 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 Unices, but today most computer programs treat this seamlessly

The filename may be encoded as defined by RFC 2231. Besides attachment, one can specify inline, or any other disposition type. Unfortunately, no name is defined for the nominal "default" disposition that corresponds to no content-disposition being present. Thus the recommended practice for generating agents is to only include filename information when it is necessary, also to avoid leaking sensitive information. If filename information has to be included, an agent should either put it in a filename= parameter or both a filename= and name= parameter. Never ever use just a name= parameter because that opens up to gratuitous interpretation of the part using an unintended disposition value.[