What makes the specificity of design? What are the skills of a designer? In order to answer to these questions, we will be looking to design as it used to be practiced, and the goals it was assigned. We are going to look for commonalities that make the different design practices bear the same name. Design is able to make the familiar unfamiliar and the unfamiliar familiar. This sentence is the red thread that will help us to discover how the designer distinguish himself from the other participants to the co-creation, what differentiates expert design from diffuse design in Manzini’s words.