RELAX NG (Regular Language for XML Next Generation) is a schema language for XML documents, based on Murata Makoto's RELAX and James Clark's TREX. The .rnc file extension specifically denotes the Compact Syntax version of this language. Unlike the standard RELAX NG syntax which is itself written in XML (using the .rng extension), the compact syntax is a non-XML, text-based notation designed to be much more human-readable and easier to author manually. It uses a concise, mathematical notation that resembles regular expressions or BNF-style grammars. This format is widely utilized in high-end document engineering and by major web standards bodies to define the structure, elements, and attributes allowed within specific XML vocabularies. Because it is a plain text format, it is exceptionally well-suited for version control systems and can be easily edited in any standard text editor. Developers often prefer .rnc for its brevity and clarity, frequently using tools like 'Trang' to convert between the compact syntax and the XML-based syntax or other schema formats like XSD.