A synesthete experiences stable cross-sensory pairings without effort or intent. Most discover in childhood that their perception is unusual, often in a moment of surprise that other people do not share it: a classmate who disagrees that five is yellow, a friend who hears the same chord and sees nothing.
Synesthetes divide roughly into projectors, who perceive the extra sensation out in space, and associators, who experience it in the mind’s eye. Both report the same consistency, which is what separates the trait from learned association or metaphor.
Estimates put synesthetes at a few percent of the population, with many unaware the trait has a name. Musicians turn up often among self-described synesthetes; our notes on famous cases cover the documented ones.