All digital documents contain markup in some form or other. Most of today’s markup systems are based on SGML (Standard Generalized Markup Language), or its subset, XML (Extensible Markup Language). Despite their many strengths, SGML/XML-based markup systems also have certain limitations. These limitations cause difficulties in representing and processing many kinds of documents. MLCD aims to lay the foundations for markup systems eliminating or minimizing these difficulties, while retaining the advantages of SGML/XML-based markup.