Synaesthetes often experience correspondences between the shades of color, tones of sounds, and intensities of tastes that provoke alternate sensations. For instance, a synaesthete may see a more intense red as the pitch of a sound gets higher, or a smoother surface might make one taste a sweeter taste. These experiences are not metaphorical or merely associations; rather, they are involuntary and are consistent throughout life, although some young synaesthetes seem to lose their ability by or during adulthood.
Synaesthesia can even occur when one of the senses no longer functions properly, e.g., a person who can see colours when words are spoken can still see the colours if he becomes blind in later life.
Two of the most common forms of synaesthesia are seeing sound or seeing letters and numbers in color.
Richard Cytowic wrote a pop-psych book about this condition entitled The Man Who Tasted Shapes.
In synaesthesia's most common form (Grapheme-color synaenesthesia), individual letters of the alphabet, as well as numbers, are "shaded" or "tinged" with a color. The alphabet color pattern is different for every individual. Many synaesthetes report that they were unaware their abilities were special or unusual until they realized other people didn't have them
Nick Carraway, the narrator of F. Scott Fitzgerald's The Great Gatsby, recounts "yellow cocktail music" playing at one of Gatsby's parties.
Ludwig van Beethoven considered B minor to be "the black key," and Franz Schubert viewed E minor as like "a maiden robed in white and with a rose-red bow on her breast." In such cases of long-dead people, it is difficult to tell whether they were describing their synesthesia or using figures of speech.