The 100% rule. If you are the piano player and you're the only one playing, you are at 100%. You're playing it all. If you add a guitar, you just halved your contribution. You are 50% and the guitar is 50%. Add a drummer and a bass player, and you become 25%. What this means is that each instrumentalist plays a little less with the addition of other instruments, and no one plays 'all out'.
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