The Ewald sphere construction. The X-ray beam is along the X axis and the Z axis is the rotation axis. The origin of the reciprocal lattice lies at the point that the X-ray beam exits the Ewald sphere (Q) and the crystal is located at the centre of the sphere (O). The crystal is oriented so that the a axis lies along the X axis. The reciprocal-lattice plane h = 1 is shown. Each diffraction spot on the detector can be mapped back to the equivalent scattering vector in reciprocal space. One such scattering vector is shown as a bold line from the reciprocal-lattice origin (Q) to the surface of the Ewald sphere.