In this paper, we present a methodology to express, in a formal way, the requirements of products belonging to a product line. We relied on a formalism allowing the representation of variabilities at the family level and the instantiation of them in order to move to the requirements of a single product. The proposed methodology also allows the formalization of the family constraints to be taken into account for the construction of the products belonging to it, along with the verification of the compliance to those constraints of a single product requirements document. This approach is promising due to its simplicity and effectiveness for being supported by automatic tools.