A prototype of a new double crystal monochromator (DCM) has been designed and developed for the second crystallography beamline (XRD2) at ELETTRA. The new device has to cover the 8–35 keV X-ray range. Since the corresponding diffraction angles are quite small, the choice has been to design a DCM with a fixed distance between the two crystals. As a consequence, the output beam has a small vertical displacement during the scan. This movement is compensated by means of an upstream mask, vertically moving and cutting the input beam at different heights. The movement of the mask is driven by a mechanism linked to the primary rotation of crystals (self-compensation), without any additional motor and following the displacement law required for compensation. The principle, the mechanism and the general mechanical concept of the device will be described in this paper.