In the model, each slice of cheese represents a safety barrier or precaution relevant to a particular hazard. For example, if the hazard were wrong-site surgery, slices of the cheese might include conventions for identifying sidedness on radiology tests, a protocol for signing the correct site when the surgeon and patient first meet, and a second protocol for reviewing the medical record and checking the previously marked site in the operating room. Many more layers exist. The point is that no single barrier is foolproof. They each have "holes"; hence, the Swiss cheese. For some serious events (e.g., operating on the wrong site or wrong person), even though the holes will align infrequently, even rare cases of harm (errors making it "through the cheese") will be unacceptable.
While the model may convey the impression that the slices of cheese and the location of their respective holes are independent, this may not be the case. For instance, in an emergency situation, all three of the surgical identification safety checks mentioned above may fail or be bypassed. The surgeon may meet the patient for the first time in the operating room. A hurried x-ray technologist might mislabel a film (or simply hang it backwards and a hurried surgeon not notice), "signing the site" may not take place at all (e.g., if the patient is unconscious) or, if it takes place, be rushed and offer no real protection. In the technical parlance of accident analysis, the different barriers may have a common failure mode, in which several protections are lost at once (i.e., several layers of the cheese line up).
In health care, such failure modes, in which slices of the cheese line up more often than one would expect if the location of their holes were independent of each other (and certainly more often than wings fly off airplanes) occur distressingly commonly. In fact, many of the systems problems discussed by Reason and others—poorly designed work schedules, lack of teamwork, variations in the design of important equipment between and even within institutions—are sufficiently common that many of the slices of cheese already have their holes aligned. In such cases, one slice of cheese may be all that is left between the patient and significant hazard.