It's where the sensory pathways in the brain "intersect", so to speak. For example, some people see letters and numbers in color. This is just one of many forms where various senses are "combined". My son sees all numbers in specific colors. We first learned this his Sophomore year in high school. He was given his varsity football jersey - #52. He told us that he hoped to keep this number throughout the next years as it was his favorite colors, red and blue. We didn't get it at all and thought he was being his usual quirky self. He was serious. (and that explained why he had so much difficulty in pre-K and K with the "color-by-number" math worksheets. He took advanced math for college credit in HS, but math was always interesting for him due to the synesthesia.) He also has a very tactile experience with music. -has played various musical instruments, beginning from age 6. -violin, cello, trumpet, percussion, and guitar. He can "feel" certain sounds, pitches, chords, etc. It is just odd....