In this paper, we make a tentative study on the strategic choice of transaction patterns for the detailed design drawings of auto parts in the Japanese auto industry. Using the available taxonomy of transaction patterns from previous studies - the Drawing-Supplied (DS) system, the Drawing-Entrusted (DE) system and the Drawing-Approved (DA) system - we further explore the conditions under which these patterns are chosen to achieve the efficiency of interfirm cooperation in the new product development process. The inquiry into why diverse patterns can still be observed even within a transaction dyad (one automaker and one supplier) motivates us to relate the architectural attributes of auto parts to the outsourcing decision for their detailed design. Results of our empirical study show that the functional modularity of auto parts had a positive effect on the outsourcing of design drawings.