Ah, the Rach argument to end all Rach arguments... awhile ago, someone said they are double jointed and for that reason they could reach a larger span, close to Rach's. Someone else said Beethoven's hands were stubby with fleshy hands... the first statement I challenge: every one of my fingers and both of my thumbs are double jointed and I can barely span a 9th with that added advantage. I find that when I play octaves with harmonic voices within (like a simple triad with the octave added) repeatedly (for example, if I read through any of Rach's pieces) my fingers get stuck in their double jointed positions and become extremely painful. As for the Beethoven comment... this explains why his music is easy for me. Being very Russian, I have peasant-stock hands with stubby fingers and fleshy palms... they look a lot like that cast, actually. However, I still have to drop notes from some of his pieces and when I play through Appassionata or the third movement of the Moonlight alone, I have the same problems I get with Rach's music: painful double joints.
I'd like to know how the other double jointed one manages to use it to his advantage instead of experiencing the pain I do (I can only practice my repertoire for 45 minutes at a time to avoid injury)