Common Lisp the Language, 2nd Edition

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2.2.5. String Characters

Any character whose bits and font attributes are zero may be contained in strings. All such characters together constitute a subtype of the characters; this subtype is called string-char.

X3J13 voted in March 1989 (CHARACTER-PROPOSAL)   to eliminate the type string-char. Two new subtypes of character are base-character, defined to be equivalent to the result of the function call

(upgraded-array-element-type 'standard-char)

and extended-character, defined to be equivalent to the type specifier

(and character (not base-character))

An implementation may support additional subtypes of character that may or may not be supertypes of base-character. In addition, an implementation may define base-character to be equivalent to character. The choice of any base characters that are not standard characters is implementation-defined. Only base characters can be elements of a base string. No upper bound is specified for the number of distinct characters of type base-character-that is implementation-dependent-but the lower bound is 96, the number of standard Common Lisp characters.