This paper introduces an extractive approach to building-up a product line based on existing systems. Thereby, we focus on the analysis of common functionalities across different systems. Our commonality analysis is performed on the functional level which offers the highest reuse potential. We use the service diagram, a formally founded specification technique, for the functional specification. This allows us to perform the commonality analysis based on a formal definition of the system behavior. Concepts for the comparison of service diagrams and a methodology for building-up a service diagram of a product line are described in the paper. Additionally, since legacy systems rarely have an accurate functional specification, we present a methodology for extracting such a specification out of a given logical architecture.