We envision the use of Extensible Markup Language (XML) documents which contain markup that is defined for and used by different software modules. One motivation for this is modularity; if there is a set of markup available which is well-understood and for which there is useful software available, it is better to re-use this markup rather than re-invent it.

Such documents, containing markup from multiple independent sources, pose problems of recognition and collision. Software modules need to be able to recognize the markup (tags and attributes) which they are designed to process, even in the face of "collisions" occurring when markup intended for some other software package uses the same element type or attribute name.

These considerations require that document constructs should have universal names, whose scope extends beyond their containing document. This specification describes a mechanism, XML namespaces, which accomplishes this.

An XML namespace is a collection of names, identified by a URI, which are used in XML documents as element types and attribute names. XML namespaces differ from the "namespaces" conventionally used in computing disciplines in that the XML version has internal structure and is not, mathematically speaking, a set. These issues are discussed in .

Names from XML namespaces may appear as qualified names, which contain a single colon, separating the name into a namespace prefix and the local part. The prefix, which is mapped to a URI, selects a namespace. The combination of the universally managed URI namespace and the document's own namespace produces identifiers that are guaranteed universally unique. A mechanism is provided for defaulting the prefix to avoid clutter and improve readability.

URIs can contain characters not allowed in names, so cannot be used directly as namespace prefixes. Therefore, the namespace prefix serves as a proxy for a URI. An attribute-based syntax described below is used to declare the association of the namespace prefix with a URI; software which supports this namespace proposal must recognize and act on these declarations and prefixes.

A namespace is declared using an attribute whose prefix is xmlns as follows. Note that many of the nonterminals in the productions in this specification are defined not here but in the XML specification . When nonterminals defined here have the same names as nonterminals defined in the XML specification, the productions here in all cases match a subset of the strings matched by the corresponding ones there. Namespace declaration using attributes NSDef PrefixDef Eq SystemLiteral PrefixDef 'xmlns' (':' NCName)? NCName (Letter | '_') (NCNameChar)*An XML Name, minus the ":" NCNameChar Letter | Digit | '.' | '-' | '_' | CombiningChar | Extender The SystemLiteral in the NSDef production is a URI which functions as a namespace name to identify the namespace. The namespace name, to serve its intended purpose, should have the characteristics of uniqueness and persistence. It is not a goal that it be directly usable for retrieval of a schema (if any exists). An example of a syntax that is designed with these goals in mind is that for Uniform Resource Names . However, it should be noted that ordinary URLs can be managed in such a way as to achieve these same goals.

In the PrefixDef production, if the optional colon and NCName are provided, then that NCName gives the namespace prefix, used to associate names with this namespace in the scope of the element to which the declaration is attached.

If the colon and NCName are not provided, then the associated namespace name is that of the default namespace in the scope of the element to which the declaration is attached.

An example namespace declaration: ]]>

In XML documents conforming to this specification, some names (constructs corresponding to the nonterminal Name) may be given as qualified names, defined as follows: Qualified Name QName (Prefix ':')? LocalPart Prefix NCName LocalPart NCName The Prefix provides the namespace prefix part of the qualified name, and must be associated with a namespace URI in a namespace declaration. The LocalPart provides the local part of the qualified name.

Note that the prefix functions only as a placeholder for a namespace name. Applications should use the namespace name, not the prefix, in constructing names whose scope extends beyond the containing document.

In XML documents conforming to this specification, element types are given as qualified names, as follows: Element Types and Attribute Names STag '<' QName (S Attribute)* S? '>' ETag '</' QName S? '>' EmptyElement '<' QName (S Attribute)* S? '/>'

Attribute names are given as qualified names, as follows: Attribute Attribute QName Eq AttValue Prefix Declared

The namespace prefix, unless it is xml or xmlns, must have been declared in a namespace declaration. The namespace prefixes xml and xmlns are reserved, and considered to have been implicitly declared. Prefixes beginning with the three-letter sequence x, m, l, in any case combination, are reserved for use by XML and XML-related specifications.

Element names and attribute types are also given as qualified names when they appear in declarations in the DTD: Qualified Names in Declarations doctypedecl '<!DOCTYPE' S QName (S ExternalID)? S? ('[' (markupdecl | PEReference | S)* ']' S?)? '>' elementdecl '<!ELEMENT' S QName S contentspec S? '>' cp (QName | choice | seq) ('?' | '*' | '+')? Mixed '(' S? '#PCDATA' (S? '|' S? QName)* S? ')*' | '(' S? '#PCDATA' S? ')' AttlistDecl '<!ATTLIST' S QName AttDef* S? '>' AttDef S QName S AttType S DefaultDecl

The namespace declaration is considered to apply to the element where it is specified and to all elements within the content of that element, unless overridden by another namespace declaration with the same PrefixDef part: Frobnostication Moved to here. ]]>

Multiple namespace prefixes can be declared as attributes of a single element, as shown in this example: Cheaper by the Dozen 1568491379 ]]>

A default namespace is considered to apply to the element where it is declared (if that element has no namespace prefix), and to all elements with no prefix within the content of that element. Note that default namespaces do not apply directly to attributes. The namespace of an unprefixed attribute is a function of the type of the element to which it is attached, and to the namespace (if any) of that element. For details, see .

Moved to here.

A larger example of namespace scoping: 1568491379

This is a funny book!

The default namespace, once declared, may be overridden:

Name	Origin	Description
Huntsman	Bath, UK	BitterFuggles Wonderful hop, light alcohol, good summer beer Fragile; excessive variance pub to pub

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Names in XML documents which conform to this specification are element types and attribute names which match the production for QName and which satisfy the "Namespace Constraints" in this document.

An XML document conforms to this specification if all other tokens in the document which are required, for XML conformance, to match the XML production for Name, match this specification's production for NCName, and in which elements have attributes with unique expanded names.

The effect of conformance is that in such a document:

All element types and attribute names contain either zero or one colon.

Strictly speaking, attribute values declared to be of types ID, IDREF(S), ENTITY(IES), and NOTATION are also Names, and thus should be colon-free. However, the declared type of attribute values is in principle only available in documents which have been validated. Thus, in well-formed XML documents, there can be no assurance that the contents of attribute values have been checked for conformance to this specification.