A design rationale is a representation of the reasoning behind a design concept, explaining why the solution is designed the way it is. This makes design rationale a critical part of concept development. However, there is little exploration on how to build a design rationale. This study sheds light on professional designers’ reasoning in conceptual design, as we examine how design rationales for different concepts are built based on a longitudinal study in the context of two design studios. Particularly the study provides insight into how a design rationale is initiated, matured and finalized.@en