RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML documents. It is a powerful, user-friendly, and highly flexible alternative to XML Schema (XSD) and Document Type Definitions (DTD). The RNG format is defined by the OASIS RELAX NG Technical Committee and is also an ISO/IEC international standard. Unlike other schema languages that can be overly verbose or restrictive, RELAX NG uses a pattern-based approach to define the structure, content, and data types of XML documents. It is particularly valued for its ability to handle complex XML structures with ease and its strong support for modularity. Developers use RNG files to validate that an XML document conforms to a specific set of rules, ensuring data integrity and consistency in data exchange processes. Because it is based on regular tree grammars, it provides a mathematically sound way to describe XML vocabularies, making it a preferred choice for many high-end publishing and data-interchange systems.