We envision applications of Extensible Markup Language (XML) where a single XML document may contain elements and attributes (here referred to as a "markup vocabulary") that are defined for and used by multiple software modules. One motivation for this is modularity; if such a markup vocabulary exists 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 multiple markup vocabularies, pose problems of recognition and collision. Software modules need to be able to recognize the 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 an IRI reference, 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 .

IRI references which identify namespaces are considered identical if and only if they are exactly the same character-for-character. Case differences and escaping differences (including case differences in escape sequences) are therefore significant. Note that IRI references which are not identical in this sense may in fact be functionally equivalent. Examples include IRI references which differ only in case or escaping , or which are in external entities which have different effective base URIs.

The empty string, though it is a legal IRI reference, cannot be used as a namespace name.

The use of relative IRI references, including same-document references, in namespace declarations is deprecated. Future W3C specifications will define no interpretation for them.

Names from XML namespaces may appear as qualified names, which may contain a single colon separating the name into a namespace prefix and a local part. The prefix, which is mapped to an IRI reference, selects a namespace. The combination of the universally managed IRI namespace and the document's own namespace produces identifiers that are universally unique. Mechanisms are provided for prefix scoping and defaulting.

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

A namespace is declared using a family of reserved attributes. Such an attribute's name must either be xmlns or have xmlns: as a prefix. These attributes, like any other XML attributes, may be provided directly or by default.

The attribute's value, an IRI reference, is the namespace name identifying 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.

If the attribute name matches PrefixedAttName, then the NCName gives the namespace prefix, used to associate element and attribute names with the namespace name in the attribute value in the scope of the element to which the declaration is attached. The attribute value may be an empty string.

If the attribute name matches DefaultAttName, then the namespace name in the attribute value is that of the default namespace in the scope of the element to which the declaration is attached. The attribute value may be an empty string. Default namespaces and overriding of declarations are discussed in .

An example namespace declaration, which associates the namespace prefix edi with the namespace name http://ecommerce.org/schema:

Though they are not themselves reserved, it is inadvisable to use prefixed names whose LocalPart begins with the letters x, m, l, as these names would be reserved if used without a prefix.

In XML documents conforming to this specification, some names (constructs corresponding to the nonterminal Name) may be given as qualified names, defined as follows:

The Prefix provides the namespace prefix part of the qualified name, and must be associated with a namespace IRI reference 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:

An example of a qualified name serving as an element type:

Attributes are either namespace declarations or their names are given as qualified names:

An example of a qualified name serving as an attribute name:

This constraint may lead to operational difficulties in the case where the namespace declaration attribute is provided, not directly in the XML document entity, but via a default attribute declared in an external entity. Such declarations may not be read by software which is based on a non-validating XML processor. Many XML applications, presumably including namespace-sensitive ones, fail to require validating processors. For correct operation with such applications, namespace declarations must be provided either directly or via default attributes declared in the internal subset of the DTD.

Element names and attribute types are also given as qualified names when they appear in declarations in the DTD:

This specification applies to XML 1.1 documents. To conform to this specification, a document must be well-formed according to the XML 1.1 specification.

In XML documents which conform to this specification, element types and attribute names must match the production for QName and must satisfy the "Namespace Constraints". All other tokens in the document which are required, for XML 1.1 well-formedness, to match the XML production for Name, must match this specification's production for NCName.

A document is namespace-well-formed if it conforms to this specification.

It follows that in a namespace-well-formed document:

In addition, a namespace-well-formed document may also be namespace-valid.

A namespace-well-formed document is namespace-valid if it is valid according to the XML 1.1 specification, and all tokens other than element types and attribute names which are required, for XML 1.1 validity, to match the XML production for Name, match this specification's production for NCName.

It follows that in a namespace-valid document:

Some characters are disallowed in URI references, even if they are allowed in XML; the disallowed characters, according to and , are the control characters #x0 to #x1F and #x7F, space #x20, the delimiters '<' #x3C, '>' #x3E and '"' #x22, the unwise characters '{' #x7B, '}' #x7D, '|' #x7C, '\' #x5C, '^' #x5E and '`' #x60, as well as all characters above #x7F.

An IRI reference is a string that can be converted to a URI reference by escaping all disallowed characters as follows:

This section will not be present in the final version of this specification. It contains productions that will appear in the XML 1.1 specification and are different from those in XML 1.0.