  In view of increased interest in object-oriented systems for describing
coordinate information, we present a description of the data model used by the
Starlink AST library. AST provides a comprehensive range of facilities for
attaching world co-ordinate systems to astronomical data, and for retrieving
and interpreting that information in a variety of formats, including FITS-WCS.
AST is a mature system that has been in use for more than 17 years, and may
consequently be useful as a means of informing development of similar systems
in the future.
