Form features present a rather attractive building block in computer-aided design environments for a variety of applications, providing the embodiment of engineering semantics in specific part shape. In this paper we address the issue of volume feature behaviour throughout interaction phenomena. First, the fundamental concept of form feature as expression of the morphology of a model, is presented. A description of the feature properties, attributes and general constraints that are relevant for our purpose is attempted. In order to develop sound validity conditions for the various classes of form features, feature definitional entities are also introduced. A comprehensive definition of interaction among features is given that encompasses both adjacent and intersecting features. We use a structured cellular modeling scheme to capture both the morphology of features and the interactions among them. Within this framework, a thorough analysis of feature interactions is performed that expl...