Zebra is the American medical slang for arriving at a surprising, often exotic, medical diagnosis when a more commonplace explanation is more likely. It is shorthand for the aphorism coined in the late 1940s by Theodore Woodward, professor at the University of Maryland School of Medicine, who instructed his medical interns: "When you hear hoofbeats behind you, don't expect to see a zebra". Since horses are common in Maryland while zebras are relatively rare, logically one could confidently guess that an animal making hoofbeats is probably a horse. By 1960, the aphorism was widely known in medical circles.