"Many people think the knuckle of the finger rests directly behind the crease that represents the base of the finger right under the bottom finger pad. Now turn your hand around. That main knuckle is SIGNIFICANTLY lower on the hand. In fact it’s below the upper pad of the palm that curve below all the fingers. A lot more of your finger resides inside the palm/body than you might think. Understanding that is a big part of figuring out hands."
tom's mad blog