Our objective is to propose an instrument that allows assessing the clarity of the writing of legal texts. Based on the interdisciplinary work, we develop research based on a multi-method approach and with an intraparadigmatic orientation. For the instrument creation, we used three complementary inputs: in-depth interviews with lawyers-teachers, surveys of lawyers and a documentary review, in addition to our knowledge on the subject. The central result of the research is a descriptive assessment scale that allows plain writing to be assessed in the legal field. This instrument was validated in different instances and by specialists from different areas. In conclusion, we offer a useful evaluation resource that meets the rigorous criteria of the design of instruments to evaluate performance, such as, in this case, the production of clear writings in the legal field.