U+0000 (Null) is the only character that is not permitted in any XML 1.1 document. Encoding detection The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings". Unicode itself defines encodings that cover the entire repertoire; well-known ones include UTF-8 (which the XML standard recommends using, without a BOM) and UTF-16. There are many other text encodings that predate Unicode, such as ASCII and various ISO/IEC 8859; their character repertoires are in every case subsets of the Unicode character set. XML allows the use of any of the Unicode-defined encodings and any other encodings whose characters also appear in Unicode. XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used. Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser (and in some cases not even UTF-16, even though the standard mandates it to also be recognized). Escaping XML provides escape facilities for including characters that are problematic to include directly. For example: The characters "<" and "&" are key syntax markers and may never appear in content outside a CDATA section. It is allowed, but not recommended, to use "<" in XML entity values. Some character encodings support only a subset of Unicode. For example, it is legal to encode an XML document in ASCII, but ASCII lacks code points for Unicode characters such as "é". It might not be possible to type the character on the author's machine. Some characters have glyphs that cannot be visually distinguished from other characters, such as the non-breaking space (&#xa0;) " " and the space (&#x20;) " ", and the Cyrillic capital letter A (&#x410;) "А" and the Latin capital letter A (&#x41;) "A". There are five predefined entities: &lt; represents "<"; &gt; represents ">"; &amp; represents "&"; &apos; represents ""; &quot; represents ''. All permitted Unicode characters may be represented with a numeric character reference. Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as &#20013; or &#x4e2d;. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as I &lt;3 J&#xF6;rg. &#0; is not permitted because the null character is one