range (the first 128 code points of Unicode) to represent, or reference, any Unicode character, regardless of whether the character being represented is directly available in the document's encoding. These special sequences are character references. Character references that are based on the referenced character's UCS or Unicode code point are called numeric character references. In HTML 4 and in all versions of XHTML and XML, the code point can be expressed either as a decimal (base 10) number or as a hexadecimal (base 16) number. The syntax is as follows: Character U+0026 (ampersand), followed by character U+0023 (number sign), followed by one of the following choices: one or more decimal digits zero (U+0030) through nine (U+0039); or character U+0078 ("x") followed by one or more hexadecimal digits, which are zero (U+0030) through nine (U+0039), Latin capital letter A (U+0041) through F (U+0046), and Latin small letter a (U+0061) through f (U+0066); all followed by character U+003B (semicolon). Older versions of HTML disallowed the hexadecimal syntax. The characters that comprise a numeric character reference can be represented in every character encoding used in computing and telecommunications today, so there is no risk of the reference itself being unencodable. There is another kind of character reference called a character entity reference, which allows a character to be referred to by a name instead of a number. (Naming a character creates a character entity.) HTML defines some character entities, but not many; all other characters can only be included by direct encoding or using NCRs. Restrictions The Universal Character Set defined by ISO 10646 is the "document character set" of SGML, HTML 4, so by default, any character in such a document, and any character referenced in such a document, must be in the UCS. While the syntax of SGML does not prohibit references to invalid or unassigned code points, such as &#xFFFF;, SGML-derived markup languages such as HTML and XML can, and often do, restrict numeric character references to only those code points that are assigned to characters. Restrictions may also apply for other reasons. For example, in HTML 4, &#12;, which is a reference to a non-printing "form feed" control character, is allowed because a form feed character is allowed. But in XML, the form feed character cannot be used, not even by reference. As another example, &#128;, which is a reference to another control character, is not allowed to be