The measurement of the silicon lattice parameter requires combining optical and X-ray interferometry. The measurement is carried out by relating the displacement of a crystal, measured by an optical interferometer, to the number of lattice planes, counted by an X-ray interferometer. During the measurement, the crystal motion must be controlled over six degrees of freedom with atomic-scale accuracy and sensitivity: 1 pm for the longitudinal position, 1 nm for the two transverse directions, 1 nrad for the pitch and yaw rotations, 1 urad for the roll one. These performances required the development of innovative measurement and control technologies. Currently, the relative accuracy of the measurement is equal to 2 x 10^-9