Studies have shown that students learning to write in engineering fields struggle to integrate subject matter and communication expertise, and that STEM faculty's communication knowledge often remains tacit, rather than being explicitly taught to students. Here we show a method for eliciting and revealing tacit communication knowledge using what we call disciplinary reasoning diagrams. We offer diagrams we have developed for Materials Science and Engineering, Brain and Cognitive Science, proof-based Mathematics, and Computer Systems, and explain how they function as instructional tools that can help students integrate knowledge domains from STEM and from writing, and to scaffold their ability to think critically and communicate effectively in their field.