Members of the OASIS TREX Technical Committee announced their decision to integrate TREX (Tree Regular Expressions for XML) and RELAX (REgular LAnguage description for XML) to collaborate on a unified lightweight specification for validating XML-based languages. They renamed their work RELAX NG.

RELAX was initially developed at the Information Technology Research and Standardization Center (INSTAC) in Japan, which advances Japanese national standards for XML under the auspices of the Japanese Standard Association (JSA). TREX was created by James Clark, widely regarded as one of the most prolific contributors to the field of structured information standards. Clark decided to continue development of his schema language at the OASIS XML interoperability consortium in March 2001.

According to the OASIS technical committee, the specification offers a middle ground that will make RELAX NG a useful tool for many developers. The team is interested in facilitating conversion among DTDs, XML Schema and RELAX NG.