In this paper, we present a study on the role of verbal communication in reaching shared understanding during idea generation in design teams. More specifically, our goal is to investigate specific communication actions designers take in different communication modalities that lead to shared understanding. In order to realize this goal, we conducted a quasi-experimental with 18 participants, who designed in groups of 3. The control groups communicated freely, whereas the test groups were not allowed to use verbal communication. We identified and analyzed communication actions that represent agreement in four different modalities: gestural, verbal, graphical and textual communication. The results suggest that when verbalization is allowed, groups rely on concepts that are generated primarily by individuals, and do not negotiate the type of shared understanding that leads to the co-creation of new concepts. When verbalization is not allowed, they are more likely to build on and advance concepts that are generated by individuals to construct that type of shared understanding, and conceptualize collectively.