Maintaining and reusing parallel numerical applications is not an easy task. We propose an OO design which enables very good code reuse for both sequential and parallel linear algebra applications. A linear algebra class library called LAKe is implemented using our design method. We show how the same code is used to implement both the sequential and the parallel version of the iterative methods implemented in LAKe. We show that polymorphism is insufficient to achieve our goal and that both genericity and polymorphism are needed. We propose a new design pattern as a part of the solution. Some numerical experiments validate our approach and show that efficiency is not sacrified.