This stuff is so cool.
Knuckles are the rounded distal (distant from the center of body) end of the metacarpals number 2 through 5… The phalanges (bones of the finger) are boxes at the base, articulating on a spool-shaped surface – thus they hinge rather than allow a broad range of motion. The length of the middle finger is equal to the distance across the metacarpals 2 through 5… The proximal phalanx is roughly equal to the middle and distal phalanges… The thumb is close to (but not exactly) 90 perpendicular to the rest of the structure.
— spence