Arachnodactyly ("spider fingers") or achromachia, is a condition in which the fingers are abnormally long and slender in comparison to the palm of the hand. It can be present at birth or develop in later life. In some cases, all or a few fingers may be bent backwards by 180 degrees.
It is possible that composer Sergei Rachmaninoff's abnormally large reach on a piano was a result of arachnodactyly due to Marfan syndrome. However, this proposal remains unconfirmed, as the pianist exhibited no other signs of the disease.