X2Rel: An XML relation language with formal semantics. X2Rel is a new language to express binary relations between XML documents. A wide range of relations can be expressed: between different versions of the same document, between an instance and a schema document, between documents before and after a transformation, and others. The syntax of X2Rel is given in XML and its semantics is defined formally. The language relies on the well-established XML technologies of XML Schema and XSL Transformations. The paper presents the ongoing development of the language.