Sketching constitutes an essential work tool for designers. On the first hand, sketches allow to externalize ideas, being then very economic cognitively. If they help to process information very quickly, sketches are also an integral part of the thinking process, without which the designer would not be able to access originality and novelty: in particular, the fact that some sketches bring new insights to the designer seems to play an important role for the emergence of ideas. Our research project aims to clarify how architects use sketches to reach generative effects by analyzing their design strategies and the way their drawings can support these strategies. We especially focus on the role of knowledge in comparison to concepts. Three sequences of sketches were analyzed thanks to the C-K design theory: two sequences of thinking sketches and one sequence of talking sketches. We show that most drawings refer to both knowledge and concepts. Moreover, our study reveals that architects carry out through sketching an important work of knowledge structuration. Indeed, generative effects often result from the introduction of new knowledge reordering the initial knowledge basis.