plain_text_wikipedia.txt

In computing, plain text is a loose term for data (e.g. file contents) that
represent only characters of readable material but not its graphical
representation nor other objects (floating-point numbers, images, etc.). It may
also include a limited number of characters that control simple arrangement of
text, such as spaces, line breaks, or tabulation characters (although tab
characters can "mean" many different things, so are hardly "plain"). Plain text
is different from formatted text, where style information is included; from
structured text, where structural parts of the document such as paragraphs,
sections, and the like are identified); and from binary files in which some
portions must be interpreted as binary objects (encoded integers, real numbers,
images, etc.).

The term is sometimes used quite loosely, to mean files that contain only
"readable" content (or just files with nothing that the speaker doesn't
prefer). For example, that could exclude any indication of fonts or layout
(such as markup, markdown, or even tabs); characters such as curly quotes,
non-breaking spaces, soft hyphens, em dashes, and/or ligatures; or other
things. 

In principle, plain text can be in any encoding, but occasionally the term is
taken to imply ASCII. As Unicode-based encodings such as UTF-8 and UTF-16
become more common, that usage may be shrinking.

Plain text is also sometimes used only to exclude "binary" files: those in
which at least some parts of the file cannot be correctly interpreted via the
character encoding in effect. For example, a file or string consisting of
"hello" (in whatever encoding), following by 4 bytes that express a binary
integer that is not just a character, is a binary file, not plain text by even
the loosest common usages. Put another way, translating a plain text file to a
character encoding that uses entirely different number to represent characters,
does not change the meaning (so long as you know what encoding is in use), but
for binary files such a conversion does change the meaning of at least some
parts of the file.

Files that contain markup or other meta-data are generally considered
plain-text, so long as the markup is also in directly human-readable form (as
in HTML, XML, and so on (as Coombs, Renear, and DeRose argue, punctuation is
itself markup; and no one considers punctuation to disqualify a file from being
plain text). 

The use of plain text rather than binary files, enables files to survive much
better "in the wild", in part by making them largely immune to computer
architecture incompatibilities. For example, all the problems of Endianness can
be avoided (with encodings such as UCS-2 rather than UTF-8, endianness matters,
but uniformly for every character, rather than for potentially-unknown subsets
of it).

According to The Unicode Standard,

"Plain text is a pure sequence of character codes; plain Un-encoded text is
therefore a sequence of Unicode character codes."

Styled text, also known as rich text, is any text representation containing
plain text completed by information such as a language identifier, font size,
color, hypertext links. Thus, representations such as SGML, RTF, HTML, XML,
wiki markup, and TeX, as well as nearly all programming language source code
files, are considered plain text. The particular contents is irrelevant to
whether a file is plain text. For example, an SVG file can express drawings or
even bitmapped graphics, but is still plain text. 

== Usage ==

The purpose of using plain text today is primarily independence from programs
that require their very own special encoding or formatting or file format.
Plain text files can be opened, read, and edited with countless text editors
and utilities.

A command-line interface allows people to give commands in plain text and get a
response, also typically in plain text.

Many other computer programs are also capable of processing or creating plain
text, such as countless programs in DOS, Windows, classic Mac OS, and Unix and
its kin; as well as web browsers (a few browsers such as Lynx and the Line Mode
Browser produce only plain text for display) and other e-text readers.

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 used for much e-mail.

A comment, a ".txt" file, or a TXT Record generally contains only plain text
(without formatting) intended for humans to read.

The best format for storing knowledge persistently is plain text, rather than
some binary format.

== Encoding ==

=== Character encodings ===

Before the early 1960s, computers were mainly used for number-crunching rather
than for text, and memory was extremely expensive. Computers often allocated
only 6 bits for each character, permitting only 64 characters—assigning codes
for A-Z, a-z, and 0-9 would leave only 2 codes: nowhere near enough. Most
computers opted not to support lower-case letters. Thus, early text projects
such as Roberto Busa's Index Thomisticus, the Brown Corpus, and others had to
resort to conventions such as keying an asterisk preceding letters actually
intended to be upper-case.

Fred Brooks of IBM argued strongly for going to 8-bit bytes, because someday
people might want to process text; and won. Although IBM used EBCDIC, most text
from then on came to be encoded in ASCII, using values from 0 to 31 for
(non-printing) control characters, and values from 32 to 127 for graphic
characters such as letters, digits, and punctuation. Most machines stored
characters in 8 bits rather than 7, ignoring the remaining bit or using it as a
checksum.

The near-ubiquity of ASCII was a great help, but failed to address
international and linguistic concerns. The dollar-sign ("$") was not so useful
in England, and the accented characters used in Spanish, French, German, and
many other languages were entirely unavailable in ASCII (not to mention
characters used in Greek, Russian, and most Eastern languages). Many
individuals, companies, and countries defined extra characters as needed—often
reassigning control characters, or using value in the range from 128 to 255.
Using values above 128 conflicts with using the 8th bit as a checksum, but the
checksum usage gradually died out.

These additional characters were encoded differently in different countries,
making texts impossible to decode without figuring out the originator's rules.
For instance, a browser might display ¬A rather than ` if it tried to interpret
one character set as another. The International Organisation for
Standardisation (ISO) eventually developed several code pages under ISO 8859,
to accommodate various languages. The first of these (ISO 8859-1) is also known
as"Latin-1", and covers the needs of most (not all) European languages that use
Latin-based characters (there was not quite enough room to cover them all). ISO
2022 then provided conventions for"switching" between different character sets
in mid-file. Many other organisations developed variations on these, and for
many years Windows and Macintosh computers used incompatible variations.

The text-encoding situation became more and more complex, leading to efforts by
ISO and by the Unicode Consortium to develop a single, unified character
encoding that could cover all known (or at least all currently known)
languages. After some conflict, these efforts were unified. Unicode currently
allows for 1,114,112 code values, and assigns codes covering nearly all modern
text writing systems, as well as many historical ones and for many
non-linguistic characters such as printer's dingbats, mathematical symbols,
etc.

Text is considered plain-text regardless of its encoding. To properly
understand or process it the recipient must know (or be able to figure out)
what encoding was used; however, they need not know anything about the computer
architecture that was used, or about the binary structures defined by whatever
program (if any) created the data.

Perhaps the most common way of explicitly stating the specific encoding of
plain text is with a MIME type.

For email and http, the default MIME type is "text/plain" -- plain text without
markup.

Another MIME type often used in both email and http is "text/html;
charset=UTF-8" -- plain text represented using UTF-8 character encoding with
HTML markup.

Another common MIME type is "application/json" -- plain text represented using
UTF-8 character encoding with JSON markup.

When a document is received without any explicit indication of the character
encoding, some applications use charset detection to attempt to guess what
encoding was used.

=== Control codes ===

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters
known as the "C0 set": codes originally intended not to represent printable
information, but rather to control devices (such as printers) that make use of
ASCII, or to provide meta-information about data streams such as those stored
on magnetic tape. They include common characters like the newline and the tab
character.

In 8-bit character sets such as Latin-1 and the other ISO 8859 sets, the first
32 characters of the "upper half" (128 to 159) are also control codes, known as
the "C1 set". They are rarely used directly; when they turn up in documents
which are ostensibly in an ISO 8859 encoding, their code positions generally
refer instead to the characters at that position in a proprietary,
system-specific encoding, such as Windows-1252 or Mac OS Roman, that use the
codes to instead provide additional graphic characters.

Unicode defines additional control characters, including bi-directional text
direction override characters (used to explicitly mark right-to-left writing
inside left-to-right writing and the other way around) and variation selectors
to select alternate forms of CJK ideographs, emoji and other characters.