By identifying the rationale for these changes in the different abstraction layers, our work contributes to establish a rigorous foundation for software versioning. Tradi- tional version codes are usually decided on the basis of the significance of changes between releases, but the decisions of the significances are entirely arbitrary and up to the author. On the basis of our approach, versioning numbers can be established in a rigorous standard way (e.g. v 1.2.3: 1 - software system specification number; 2 - pro- gram specification number, 3 - code number).