Interphalangeal joints

Typically abbreviated IP joint, the are the joints between the phalanges (the bones that make up the fingers or toes).  The first digit (thumb or big toe) has only two phalanges, and so only one IP joint.  Digits 2-5 have 3 phalanges, and so have a proximal IP joint (between the proximal and middle phalanx) and a distal IP joint (between the middle and distal phalanx).