The smallest chemical components of matter are atoms and molecules. Usually these components have a regular structure in solids. Such solids are called crystals (e.g. salt, quartz). Adequate radiation (X-rays, neutrons) is diffracted by crystals and changes direction. The diffracted rays contain information about the inner structure of the crystal. A device for measuring this effect is the diffractometer.

The measurement principle of diffractometry is based on the Bragg-Equation 2d·sin(θ)=λ. By irradiation of the crystal with a fixed wave θ with given lattice plane d the rays are diffracted in the direction of 2θ relatively to the incoming ray. This is the principle of coherent elastic neutron diffraction - at least in solids. When studying fluids and gasses, an inelastic component also plays a role.

Depending on the material and question (normal or magnetic solid, fluid, polymer etc.), different diffraction methods are used.

Lattice plane: parallel, equally spaced plane in a lattice with similar atomic arrangement.
Coherent elastic: no energy is transferred to the solid by the neutron impact.
Inelastic: the parts of the system change energy and momentum.