Abstract: Modular prosthesis kits and systems are disclosed. The kit includes a plurality of stems having a bore therein and a plurality of necks having a tapered distal end adapted to taper lock in the bore of one of the necks. Each neck includes a stem registration element on the distal end of the neck, and each stem includes a neck registration element in an end portion of the bore. The stem registration elements of certain necks and the neck registration elements of certain stems are configured to prevent certain necks from taper locking with certain stems in at least one position, and the stem registration elements of certain necks and the neck registration elements of certain stems cooperate to permit taper locking of certain necks with certain stems.