What is synesthesia?

People with synesthesia, called synesthetes, experience a blending of senses that is consistent and involuntary. One of the most common forms is grapheme–colour synesthesia, in which letters or numbers appear to have specific colours. Others may taste words, see music as shapes, or feel sounds on their skin. Estimates suggest around 2 to 4 percent of people have some form of synesthesia, and it tends to run in families. Artists and musicians such as Wassily Kandinsky and Pharrell Williams have described synesthetic experiences.