In a scanning x-ray interferometer cut from a silicon crystal the fringe period is equal to the lattice spacing selected (about 0.2 nm), which is well known in the unit of length. It is a practical instrument for high resolution measurements of small displacements, such as occur in investigations of high quality optical surfaces. In combination with an optical interferometer its fine scale having atomic distances as graduation marks, can be used for testing and for calibrating conventional interpolation techniques.