 Grapheme–color synesthesia is one of the most common forms of synesthesia and, because of the extensive knowledge of the visual system, one of the most studied.While it is extremely unlikely that any two synesthetes will report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., "A" is likely to be red).Early studies argued that grapheme–color synesthesia was not due to associative learning, such as from playing with colored refrigerator magnets.