Care should be taken, in particular where multi-byte character sets are used, that the syntactic distinctions required by an SGML processor are preserved. Where a suitable character set is defined by a national or international standard, local customization should implement the standard rather than inventing yet another non-standard character set. In principle, however, for local processing, encoders should use whatever character set they find convenient. When the characters in a text exist in the local character set, the appropriate character codes should be used to represent them. Virtually all computer systems provide at least the 52 letters of the standard Latin alphabet, the ten decimal digits, and some basic punctuation marks.
Oriental scripts pose particular problems because of the size of their character repertoires.
As noted above, full use of a local character set may require that the SGML declaration be modified to define all the characters used as legal SGML characters. Characters Not Available Locally Characters not available in the local character set should usually be encoded using SGML entity references . Entities are included in SGML documents by entity references . Lists of suggested names for all the characters and printers' symbols used by most modern European languages have been published by ISO and others. As noted above, standard entity names have been defined for most characters used by languages written in the Latin alphabet, and for some other scripts, including Greek, Cyrillic, Coptic, and the International Phonetic Alphabet.