Compatibility deals with the stimuli and responses that are consistent with human expec-
tations. Spatial, movement, or conceptual relationships with stimuli and responses are all
types of compatibility. For example, if a system component moves up and down, a control
for that movement should move up and down (movement). If, as in the introductory para-
graph on Three Mile Island, there are pairs of controls and displays that go together, they
should have the same spatial orientation. Pair A should be on the left and pair B on the
right (spatial). If water flows through five pumps in sequence, the controls for the pumps
should be in the same sequence as the flow through the pumps (spatial and conceptual).
Red means stop or danger; green means go or safe (conceptual).