A model for describing form feature geometry is presented. The model treats form features as volumes enveloped by entry/exit and depth boundaries. The geometric characteristics of a feature are decided by the degree of accessibility to its volume, its boundary type, its exit boundary status and its form variation with respect to its depth axis. A hierarchical structure for form features classification is also presented. Based on their geometric characteristics, features are classified into categories, classes and sub-classes. These can be followed by secondary forms to fully describe compound features. The feature description method together with the associated hierarchy provide a very flexible model for feature description and classification. The model is of direct relevance to many of the CAD/CAM activities and contain concepts that can help to ease their integration.