For a given incidence angle of X-rays onto a surface, only the intersections of the crystal truncation rods with the Ewald sphere can be observed. To measure the intensity along a CTR, the sample must be rotated in the X-ray beam so that the origin of the Ewald sphere is translated and the sphere intersects the rod at a different location in reciprocal space. Performing a rodscan in this way requires accurate coordinated motion of the sample and the detector along different axes. To achieve this motion, the sample and detector are mounted in an apparatus called a four-circle diffractometer. The sample is rotated in the plane bisecting the incoming and diffracted beam and the detector is moved into the position necessary to capture the diffracted CTR intensity.