This proposal is meant as design input to the XTM [XML Topic Maps] Data model group, and uses an EXPRESS schema with accompanying EXPRESS-G diagrams to define the data model. It follows the ISO syntax very closely, and is perhaps best seen as an interpretation of the ISO syntax, with some added opinions. . . Brief explanations of EXPRESS-G constructs: (1) boxes are entities (that is, class definitions); (2) rounded boxes with names are proxies for entities defined on other pages; (3) rounded boxes without names indicate references from other pages; (4) dotted lines represent optional properties; (5) S[0:?] means ‘a set of zero or more members’; (6) thick lines indicate inheritance relationships; (7) stars in front of property names indicate a uniqueness requirement. I recommend starting with the diagrams and only afterwards moving on to the textual schema. Note that the textual schema contains some important comments.