When a model is implemented in an object-oriented language, associations are implemented as attributes. All associations could be modeled from the start as attributes, but all attributes could be modeled as associations equally well. The modeler therefore cannot choose to use an association or an attribute only by reference to the reality underlying the model. Instead, the modeler chooses one or the other for its expressive power. In some contexts, these modeling decisions are independent of whether the modeler is engaged in analysis or design. The first pattern, Stable Representations, deals with these situations. Since analysis and design serve different expressive needs, however, the decisions are likely to differ in other contexts. The second pattern, Changeable Representatins, deal with those situations.