In this qualitative study, we investigated teachers' use of proportional knowledge resources while being asked to appropriately identify if a given situation was proportional or not using clinical interview data from a large grant funded project. We found that knowledge resources related to the mathematical structure of the situation were most useful in correctly identifying the situation. Interestingly, teachers were often able to identify a mathematical expression for the situations; however, these rules did not support the correct identification of the situation as proportional or not. We discuss suggested knowledge resources for the creation of a coordination class for identifying situations that are proportional from those that are not as well as implications for teacher education and professional development around proportional reasoning.