A collimated, monochromatic X-ray beam incident on the sample generates scattered X-rays, which are imaged by a detector. The transmitted beam is usually blocked by a beamstop, resulting in a shadow in the image. The scattering vector, q, describes the change in direction of the elastically scattered X-rays, and is roughly parallel to the detector face in the small-angle approximation. The images are integrated to yield scattering intensity as a 1D function of q. The scattering intensity can also be expressed as a function of s = q/2π, which is equivalent to “resolution” or d-spacing in crystallography. Because q and s are often used interchangeably in the literature, the exact usage should be explicitly defined in any publication with SAXS data.