Complementary experiments

In physics, two experimental techniques are often called complementary if they investigate the same subject in two different ways such that two different (ideally non-overlapping) properties or aspects can be investigated. For example, X-ray scattering and neutron scattering experiments are often said to be elementary because the former reveals information about the electron density of the atoms in the target but gives no information about the nuclei (because they are to small to affect the X-rays significantly), while the latter allows to investigate the nuclei of the atoms but cannot tell you anything about their electron hulls (because the neutrons, being neutral, do not interact with the charged electrons).