A common syndrome in much of the current quantum optics and quantum computing literature is the casual switching between classical concepts (e.g., geometric rays, electromagnetic waves) and quantum concepts (e.g., state vectors, projection operators). Such ambiguous language can confuse designers not well versed in the deeper subtleties of quantum mechanics, or worse, it can lead to a flawed analysis of new designs for quantum devices. To validate that a quantum device can be constructed with the expected characteristics and that its quantum effects are correctly interpreted, a set of unambiguous design rules would be useful. In this paper we enumerate such a set of easily applied quantum rules in the hope that they might facilitate clearer communication between researchers and system developers in the field. In part, we are motivated by recently reported interferometer results that have not only led to flawed claims about disproving fundamental quantum principles, but have elicited equally flawed counter aruments from supposedly knowledgeable respondents. After one hundred years of testing Einstein's photon, it is alarming that such widespread confusion still persists. Our proposed quantum design rules are presented in a practical diagrammatic style, demonstrating their effectiveness by analyzing several interferometers that have appeared in the recent literature. Application to other quantum devices e.g., quantum ghost imaging, are also discussed. We stress that these rules are entirely quantum in prescription, being particularly appropriate for single-photon devices. Classical optics concepts e.g., refractive index, are not required since they are subsumed by our quantum rules.