A mounting mechanism for a double crystal monochromator or the like has a parallelogram based mounting mechanism in which two of the vertices of the parallelogram are fixed in position, and two vertices are free to translate back and forth in a straight line parallel to the fixed base of the parallelogram. One diffractor is mounting for pivoting at one of the fixed vertices, and the second diffractor is mounted for pivoting at an adjacent movable vertex. The surfaces of the diffractors are maintained parallel as the angle of the diffractors with respect to input and output beams to the monochromator is changed to change the wavelength being passed. The diffractor mounted at the fixed pivot may be connected to a large diameter wheel which in turn is connected by a band to a smaller diameter wheel mounted for rotation at the other fixed vertex of the parallelogram, with a pivotable arm connected to the smaller wheel to rotate therewith. Another larger diameter wheel is connected to the diffractor at the movable vertex and is connected by a band to a smaller diameter wheel at the adjacent movable vertex of the parallelogram, where the smaller wheel is connected by a slider to the pivotable arm. As the movable diffractor is translated in position, the arm pivots to cause the small wheels to move through the same angle of angular displacement as the pivotable arm. The corresponding angular displacement of the diffractors may be one half the angular displacement of the pivotable arm where the larger wheels are twice the diameter of the smaller wheels.