A set of mobile necks for inserting into the stem (2) of a hip prosthesis, in which the mobile necks are produced with different inclinations and lengths in such a way that their free ends (3i), when inserted in the stem, terminate at nine mutually equidistant points (Pi) arranged in three parallel horizontal rows (R0) in such a way that the lines joining the outermost points delineate a square (Q), the length of whose sides is approximately 15 mm, the diagonal (D) of this square coinciding with the axis (H-H) of a mobile neck (1) set at a neutral inclination, that is neither varus nor valgus, with respect to a plane (ss) essentially perpendicular to the axis of the abovementioned stem (2).
