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Base character sets: using more than one
Prerequisite Concepts

More than one base character set can be used in a document. The document character set must be assigned the meanings of all characters in all of the base character sets used. All of the base character sets used in the document character set that contain significant (LC Letter, UC Letter, Digit, or Special) characters must be used in defining the syntax-reference character set, and all significant characters in those base character sets must have their meanings assigned to syntax-reference characters. Any other base character set used to assign meanings in the document character set may be used in the syntax-reference character set, as may any base character set not used in the document character set. However, in the latter case, no "meaningful" assignments can be made to the syntax-reference characters, because there are no document characters that take on those syntax-reference character meanings.

The repetition of a public identifier in the document character set is recognized, and the previous definition of the base character set is used. Where (the same or different) base character sets assign an external character number to the same graphic character, the first assignment of the base character set is used.