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     NAME
          charsets - character set standards and internationalization

     DESCRIPTION
          This manual page gives an overview on different character
          set standards and how they were used on Linux before Unicode
          became ubiquitous.  Some of this information is still
          helpful for people working with legacy systems and
          documents.

          Standards discussed include such as ASCII, GB 2312, ISO
          8859, JIS, KOI8-R, KS, and Unicode.

          The primary emphasis is on character sets that were actually
          used by locale character sets, not the myriad others that
          could be found in data from other systems.

        ASCII
          ASCII (American Standard Code For Information Interchange)
          is the original 7-bit character set, originally designed for
          American English.  Also known as US-ASCII.  It is currently
          described by the ISO 646:1991 IRV (International Reference
          Version) standard.

          Various ASCII variants replacing the dollar sign with other
          currency symbols and replacing punctuation with non-English
          alphabetic characters to cover German, French, Spanish, and
          others in 7 bits emerged.  All are deprecated; glibc does
          not support locales whose character sets are not true
          supersets of ASCII.

          As Unicode, when using UTF-8, is ASCII-compatible, plain
          ASCII text still renders properly on modern UTF-8 using
          systems.

        ISO 8859
          ISO 8859 is a series of 15 8-bit character sets, all of
          which have ASCII in their low (7-bit) half, invisible
          control characters in positions 128 to 159, and 96 fixed-
          width graphics in positions 160en255.

          Of these, the most important is ISO 8859-1 ("Latin Alphabet
          No .1" / Latin-1).  It was widely adopted and supported by
          different systems, and is gradually being replaced with
          Unicode.  The ISO 8859-1 characters are also the first 256
          characters of Unicode.

          Console support for the other 8859 character sets is
          available under Linux through user-mode utilities (such as
          setfont(8)) that modify keyboard bindings and the EGA

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          graphics table and employ the "user mapping" font table in
          the console driver.

          Here are brief descriptions of each set:

          8859-1 (Latin-1)
               Latin-1 covers many West European languages such as
               Albanian, Basque, Danish, English, Faroese, Galician,
               Icelandic, Irish, Italian, Norwegian, Portuguese, Span-
               ish, and Swedish.  The lack of the ligatures Dutch IJ/ij,
               French œ, and old-style „German“ quotation marks was
               considered tolerable.

          8859-2 (Latin-2)
               Latin-2 supports many Latin-written Central and East
               European languages such as Bosnian, Croatian, Czech,
               German, Hungarian, Polish, Slovak, and Slovene.
               Replacing Romanian ș/ț with ş/ţ was considered tolera-
               ble.

          8859-3 (Latin-3)
               Latin-3 was designed to cover of Esperanto, Maltese,
               and Turkish, but 8859-9 later superseded it for Turk-
               ish.

          8859-4 (Latin-4)
               Latin-4 introduced letters for North European languages
               such as Estonian, Latvian, and Lithuanian, but was
               superseded by 8859-10 and 8859-13.

          8859-5
               Cyrillic letters supporting Bulgarian, Byelorussian,
               Macedonian, Russian, Serbian, and (almost completely)
               Ukrainian.  It was never widely used, see the discus-
               sion of KOI8-R/KOI8-U below.

          8859-6
               Was created for Arabic.  The 8859-6 glyph table is a
               fixed font of separate letter forms, but a proper dis-
               play engine should combine these using the proper ini-
               tial, medial, and final forms.

          8859-7
               Was created for Modern Greek in 1987, updated in 2003.

          8859-8
               Supports Modern Hebrew without niqud (punctuation
               signs).  Niqud and full-fledged Biblical Hebrew were
               outside the scope of this character set.

          8859-9 (Latin-5)
               This is a variant of Latin-1 that replaces Icelandic

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               letters with Turkish ones.

          8859-10 (Latin-6)
               Latin-6 added the Inuit (Greenlandic) and Sami (Lap-
               pish) letters that were missing in Latin-4 to cover the
               entire Nordic area.

          8859-11
               Supports the Thai alphabet and is nearly identical to
               the TIS-620 standard.

          8859-12
               This set does not exist.

          8859-13 (Latin-7)
               Supports the Baltic Rim languages; in particular, it
               includes Latvian characters not found in Latin-4.

          8859-14 (Latin-8)
               This is the Celtic character set, covering Old Irish,
               Manx, Gaelic, Welsh, Cornish, and Breton.

          8859-15 (Latin-9)
               Latin-9 is similar to the widely used Latin-1 but
               replaces some less common symbols with the Euro sign
               and French and Finnish letters that were missing in
               Latin-1.

          8859-16 (Latin-10)
               This set covers many Southeast European languages, and
               most importantly supports Romanian more completely than
               Latin-2.

        KOI8-R / KOI8-U
          KOI8-R is a non-ISO character set popular in Russia before
          Unicode.  The lower half is ASCII; the upper is a Cyrillic
          character set somewhat better designed than ISO 8859-5.
          KOI8-U, based on KOI8-R, has better support for Ukrainian.
          Neither of these sets are ISO-2022 compatible, unlike the
          ISO 8859 series.

          Console support for KOI8-R is available under Linux through
          user-mode utilities that modify keyboard bindings and the
          EGA graphics table, and employ the "user mapping" font table
          in the console driver.

        GB 2312
          GB 2312 is a mainland Chinese national standard character
          set used to express simplified Chinese.  Just like JIS X
          0208, characters are mapped into a 94x94 two-byte matrix
          used to construct EUC-CN.  EUC-CN is the most important
          encoding for Linux and includes ASCII and GB 2312.  Note

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          that EUC-CN is often called as GB, GB 2312, or CN-GB.

        Big5
          Big5 was a popular character set in Taiwan to express tradi-
          tional Chinese.  (Big5 is both a character set and an encod-
          ing.)  It is a superset of ASCII.  Non-ASCII characters are
          expressed in two bytes.  Bytes 0xa1en0xfe are used as leading
          bytes for two-byte characters.  Big5 and its extension were
          widely used in Taiwan and Hong Kong.  It is not ISO 2022
          compliant.

        JIS X 0208
          JIS X 0208 is a Japanese national standard character set.
          Though there are some more Japanese national standard char-
          acter sets (like JIS X 0201, JIS X 0212, and JIS X 0213),
          this is the most important one.  Characters are mapped into
          a 94x94 two-byte matrix, whose each byte is in the range
          0x21en0x7e.  Note that JIS X 0208 is a character set, not an
          encoding.  This means that JIS X 0208 itself is not used for
          expressing text data.  JIS X 0208 is used as a component to
          construct encodings such as EUC-JP, Shift_JIS, and ISO-
          2022-JP.  EUC-JP is the most important encoding for Linux
          and includes ASCII and JIS X 0208.  In EUC-JP, JIS X 0208
          characters are expressed in two bytes, each of which is the
          JIS X 0208 code plus 0x80.

        KS X 1001
          KS X 1001 is a Korean national standard character set.  Just
          as JIS X 0208, characters are mapped into a 94x94 two-byte
          matrix.  KS X 1001 is used like JIS X 0208, as a component
          to construct encodings such as EUC-KR, Johab, and ISO-2022-
          KR.  EUC-KR is the most important encoding for Linux and
          includes ASCII and KS X 1001.  KS C 5601 is an older name
          for KS X 1001.

        ISO 2022 and ISO 4873
          The ISO 2022 and 4873 standards describe a font-control
          model based on VT100 practice.  This model is (partially)
          supported by the Linux kernel and by xterm(1).  Several ISO
          2022-based character encodings have been defined, especially
          for Japanese.

          There are 4 graphic character sets, called G0, G1, G2, and
          G3, and one of them is the current character set for codes
          with high bit zero (initially G0), and one of them is the
          current character set for codes with high bit one (initially
          G1).  Each graphic character set has 94 or 96 characters,
          and is essentially a 7-bit character set.  It uses codes
          either 040en0177 (041en0176) or 0240en0377 (0241en0376).  G0
          always has size 94 and uses codes 041en0176.

          Switching between character sets is done using the shift

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          functions haN (SO or LS1), haO (SI or LS0), ESC n (LS2), ESC o
          (LS3), ESC N (SS2), ESC O (SS3), ESC ti (LS1R), ESC } (LS2R),
          ESC | (LS3R).  The function LSn makes character set Gn the
          current one for codes with high bit zero.  The function LSnR
          makes character set Gn the current one for codes with high
          bit one.  The function SSn makes character set Gn (n=2 or 3)
          the current one for the next character only (regardless of
          the value of its high order bit).

          A 94-character set is designated as Gn character set by an
          escape sequence ESC ( xx (for G0), ESC ) xx (for G1), ESC *
          xx (for G2), ESC + xx (for G3), where xx is a symbol or a
          pair of symbols found in the ISO 2375 International Register
          of Coded Character Sets.  For example, ESC ( @ selects the
          ISO 646 character set as G0, ESC ( A selects the UK standard
          character set (with pound instead of number sign), ESC ( B
          selects ASCII (with dollar instead of currency sign), ESC (
          M selects a character set for African languages, ESC ( ! A
          selects the Cuban character set, and so on.

          A 96-character set is designated as Gn character set by an
          escape sequence ESC - xx (for G1), ESC . xx (for G2) or ESC
          / xx (for G3).  For example, ESC - G selects the Hebrew
          alphabet as G1.

          A multibyte character set is designated as Gn character set
          by an escape sequence ESC $ xx or ESC $ ( xx (for G0), ESC $
          ) xx (for G1), ESC $ * xx (for G2), ESC $ + xx (for G3).
          For example, ESC $ ( C selects the Korean character set for
          G0.  The Japanese character set selected by ESC $ B has a
          more recent version selected by ESC & @ ESC $ B.

          ISO 4873 stipulates a narrower use of character sets, where
          G0 is fixed (always ASCII), so that G1, G2 and G3 can be
          invoked only for codes with the high order bit set.  In par-
          ticular, haN and haO are not used anymore, ESC ( xx can be
          used only with xx=B, and ESC ) xx, ESC * xx, ESC + xx are
          equivalent to ESC - xx, ESC . xx, ESC / xx, respectively.

        TIS-620
          TIS-620 is a Thai national standard character set and a
          superset of ASCII.  In the same fashion as the ISO 8859
          series, Thai characters are mapped into 0xa1en0xfe.

        Unicode
          Unicode (ISO 10646) is a standard which aims to unambigu-
          ously represent every character in every human language.
          Unicode's structure permits 20.1 bits to encode every char-
          acter.  Since most computers don't include 20.1-bit inte-
          gers, Unicode is usually encoded as 32-bit integers inter-
          nally and either a series of 16-bit integers (UTF-16) (need-
          ing two 16-bit integers only when encoding certain rare

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          characters) or a series of 8-bit bytes (UTF-8).

          Linux represents Unicode using the 8-bit Unicode Transforma-
          tion Format (UTF-8).  UTF-8 is a variable length encoding of
          Unicode.  It uses 1 byte to code 7 bits, 2 bytes for 11
          bits, 3 bytes for 16 bits, 4 bytes for 21 bits, 5 bytes for
          26 bits, 6 bytes for 31 bits.

          Let 0,1,x stand for a zero, one, or arbitrary bit.  A byte
          0xxxxxxx stands for the Unicode 00000000 0xxxxxxx which
          codes the same symbol as the ASCII 0xxxxxxx.  Thus, ASCII
          goes unchanged into UTF-8, and people using only ASCII do
          not notice any change: not in code, and not in file size.

          A byte 110xxxxx is the start of a 2-byte code, and 110xxxxx
          10yyyyyy is assembled into 00000xxx xxyyyyyy.  A byte
          1110xxxx is the start of a 3-byte code, and 1110xxxx
          10yyyyyy 10zzzzzz is assembled into xxxxyyyy yyzzzzzz.
          (When UTF-8 is used to code the 31-bit ISO 10646 then this
          progression continues up to 6-byte codes.)

          For most texts in ISO 8859 character sets, this means that
          the characters outside of ASCII are now coded with two
          bytes.  This tends to expand ordinary text files by only one
          or two percent.  For Russian or Greek texts, this expands
          ordinary text files by 100%, since text in those languages
          is mostly outside of ASCII.  For Japanese users this means
          that the 16-bit codes now in common use will take three
          bytes.  While there are algorithmic conversions from some
          character sets (especially ISO 8859-1) to Unicode, general
          conversion requires carrying around conversion tables, which
          can be quite large for 16-bit codes.

          Note that UTF-8 is self-synchronizing: 10xxxxxx is a tail,
          any other byte is the head of a code.  Note that the only
          way ASCII bytes occur in a UTF-8 stream, is as themselves.
          In particular, there are no embedded NULs (aq\0aq) or aq/aqs
          that form part of some larger code.

          Since ASCII, and, in particular, NUL and aq/aq, are unchanged,
          the kernel does not notice that UTF-8 is being used.  It
          does not care at all what the bytes it is handling stand
          for.

          Rendering of Unicode data streams is typically handled
          through "subfont" tables which map a subset of Unicode to
          glyphs.  Internally the kernel uses Unicode to describe the
          subfont loaded in video RAM.  This means that in the Linux
          console in UTF-8 mode, one can use a character set with 512
          different symbols.  This is not enough for Japanese, Chi-
          nese, and Korean, but it is enough for most other purposes.

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     SEE ALSO
          iconv(1), ascii(7), iso_8859-1(7), unicode(7), utf-8(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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