MISRA C is the most authoritative language subset for the C programming language that is a de facto standard in several industry sectors where safety and security are of paramount importance. While MISRA C is currently encoded in 175 guidelines (coding rules and directives), it does not coincide with them: proper adoption of MISRA C requires embracing its preventive approach (as opposed to the "bug finding" approach) and a documented development process where justifiable non-compliances are authorized and recorded as deviations. MISRA C guidelines are classified along several axes in the official MISRA documents. In this paper, we add to these an orthogonal classification that associates guidelines with their main rationale. The advantages of this new classification are illustrated for different kinds of projects, including those not (yet) having MISRA compliance among their objectives.