DB21 is a diffractometer for the study of the structure of crystalline biological complexes at low resolution (lower than 10Å). The method exploits the principles of contrast variation in which particular components of a macromolecular complex can be highlighted by varying the H₂O/D₂O content of the solvent. This is done by soaking crystals in the appropriate mixture. Typical objects studied are viruses, consisting of protein and nucleic acid, or the protein-detergent complexes of solubilised membrane proteins. Typical crystal sizes are 0.01 - 1.0 mm³.

Because of the limited crystal sizes available (rarely > 1.0 mm³) the instrument is very small and compact rather like an X-ray diffractometer. To obtain a good spatial resolution between diffraction spots and to separate the lowest diffraction orders from the beam-stop an incident wavelength of 7.56Å is selected using a rather novel K-intercalated graphite. For smaller unit cells a pyrolytic graphite monochromator giving neutrons of 4.6Å wavelength is also available.

The detector is of the Anger camera type allowing a resolution of 1.75 x 1.53 mm. Due to the γ-sensitivity of this detector no cadmium is used in beam defining apertures that are instead made of LiF backed with B₄C. A single crystal of Bi removes in-beam γ-radiation.