The Music Encoding Initiative (MEI) is an open-source effort to define a system for encoding musical documents in a machine-readable structure. MEI brings together specialists from various music research communities, including technologists, librarians, historians, and theorists in a common effort to define best practices for representing a broad range of musical documents and structures. The results of these discussions are formalized in the MEI schema, a core set of rules for recording physical and intellectual characteristics of music notation documents expressed as an eXtensible Markup Language (XML) schema. It is complemented by the MEI Guidelines, which provide detailed explanations of the components of the MEI model and best practices suggestions. One of the core strengths of the MEI Schema is that it allows an individual to validate an MEI file against an XML Schema to ensure the MEI file conforms to expected encodings and behaviours. To validate an MEI file you need XML validator software. XML Authoring tools, such as oXygen has built-in validation tools. There are also several command-line utilities, including xmllint and jing.