Although the RELAX NG specification was developed at roughly the same time as the W3C "XML Schema specification, the latter was arguably better known and more widely implemented in both open-source and proprietary XML parsers and editors when it became a W3C Recommendation in 2001. Since then, however, RELAX NG support has increasingly found its way into XML software, and its acceptance has been aided by its adoption as a primary schema for popular document-centric markup languages such as "DocBook, the "TEI Guidelines, "OpenDocument, and "EPUB.
RELAX NG shares with W3C XML Schema many features that set both apart from traditional "DTDs: "data typing, "regular expression support, "namespace support, ability to reference complex definitions.
By informal convention, RELAX NG schemas in the regular syntax are typically named with the "filename extension ".rng". For schemas in the compact syntax, the extension ".rnc" is used.