Design rationale is the explicit documentation of the reasons behind decisions made in designing a product or system. Typically, design rationale is captured using a combination of written reports and oral presentations. Research shows that the structure and information used to communicate rationale significantly influence human behavior. To better understand the influence that communication of design rationale has on the design process, a detailed understanding of the information and techniques used to communicate design rationale needs to be studied. This research aims to identify how engineers and designers communicate this information in written form and the implications that their communication patterns have in engineering design. Eight hundred and forty-six pages of technical engineering design reports from 28 teams representing 116 individuals were analyzed using a mixed-methods approach and then compared across project types. The data were coded into categories using a schema we developed. The findings highlight the range of clarity that designers use in their rationales to support their design actions. Instead of clear, logical reasoning trends, designers often use techniques to fill gaps in design rationale through making assumptions, inserting oneself, or redirecting focus. The results suggest a need for improving design communication in engineering education and practice, perhaps through existing design reasoning frameworks or design rationale capturing tools. By capturing design rationale clearly between human designers, human-AI systems can leverage these findings to increase human confidence in and acceptance of a design agent’s recommendations.