Anatomical diagram may be useful

[fordacious]

"A map of hand anatomy showing the metacarpal and various phalanx bones would be a nice anatomy refresher for developers--though the hand drawing in the spec just above XRHand is also very useful."

[cabanier]

I'm unsure this is necessary. The hand drawing in the spec is sufficient.

[De-Panther]

I think there's a place to improve the hand drawing in the spec.
Don't know if names is what missing there, as there are joint numbers and a table with all the names.

What missing in the hand drawing, is the coordinate system of the joints:

The native origin has its -Y direction pointing perpendicular to the skin, outwards from the palm, and -Z direction pointing along their associated bone, away from the wrist.

For tip skeleton joints where there is no associated bone, the -Z direction is the same as that for the associated distal joint, i.e. the direction is along that of the previous bone. For wrist skeleton joints the -Z direction SHOULD point roughly towards the center of the palm.

Adding coordinate axes arrows to the drawing, for each joint on both hands, would be much more easy to understand, rather than trying to imagine this explanation.