XML Schema is an XML based alternative to DTD.

An XML schema describes the structure of an XML document.

The XML Schema language is also referred to as XML Schema Definition (XSD).

---

What You Should Already Know

Before you study the XML Schema Language, you should have a basic understanding of XML and XML Namespaces. It will also help to have some basic understanding of DTD.

If you want to study these subjects first, please visit our XML tutorial and our DTD tutorial.

What is an XML Schema?

The purpose of an XML Schema is to define the legal building blocks of an XML document, just like a DTD.

An XML Schema:

• defines elements that can appear in a document
• defines attributes that can appear in a document
• defines which elements are child elements
• defines the order of child elements
• defines the number of child elements
• defines whether an element is empty or can include text
• defines data types for elements and attributes
• defines default and fixed values for elements and attributes

XML Schemas are the Successors of DTDs

We think that very soon XML Schemas will be used in most Web applications as a replacement for DTDs. Here are some reasons:

• XML Schemas are extensible to future additions
• XML Schemas are richer and more useful than DTDs
• XML Schemas are written in XML
• XML Schemas support data types
• XML Schemas support namespaces

XML Schema is a W3C Recommendation

XML Schema was originally proposed by Microsoft, but became an official W3C recommendation in May 2001.

The specification is now stable and has been reviewed by the W3C Membership.

For a full overview of W3C Activities and Status, visit our W3C tutorial.

An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself. These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as uniqueness and referential integrity constraints. There are languages developed specifically to express XML schemas. The Document Type Definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas. Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG. The mechanism for associating an XML document with a schema varies according to the schema language. The association may be achieved via markup within the XML document itself, or via some external means.