Abstract: With the invention a modular system for the mounting of shaft prostheses is shown which have a coupling which can be fixed within a predetermined angular range between the prosthesis stem and the prosthesis head, which can be fixed by an apparatus in the prosthesis stem. Through a modular system with combinable stems and prosthesis heads of differing sizes for prostheses and test prostheses which are constructionally alike externally and in the position of the point of rotation, a variety of test prostheses and shaft prostheses arises. The test prostheses have an apparatus in the prosthesis head which permit the fixing of the head in an ideal position with a controllable function in a stem inserted in a bone. This position between the head and the stem is preserved when the test prosthesis is extracted and transferred to a mounting apparatus, in which a shaft prosthesis which is built up of analogous parts is brought into the same position and fixed.