Abstract: The invention relates to a method for determining a geometrical object for modeling the geometry of a metal sheet forming stage (1) in a CAD system. According to said method, an operator is defined which links a first geometrical model with a second geometrical model. The link is associated with a method for physically modeling a treating process which transfers a forming stage (1) from a corresponding first state into a second state. When the first geometrical model is modified, the second geometrical model is automatically updated in accordance with the physical modeling concept. The physical modeling concept for forming stages is thereby integrated into the static geometrical model environment of a CAD system. The physical modeling method is a method for calculating a border line (3) of a forming stage in an initial state prior to a forming process from a geometry of the border line (3) in a resulting state after the forming process.