this paper is to give precise semantics to the feature construct for SMV. Using such semantics, we can explore in what circumstances the feature construct is independent of the syntax of the base program to which the feature is applied, and other properties of features. This allows us to explore when features commute with each other, and, more generally, to explore what classes of features are `interaction-friendly' with respect to other classes. It is also a step towards being able to verify the feature itself, independently of the system to which it is added.