Since X-rays are electromagnetic waves it is to be expected that they will show the phenomenon of diffraction, and in fact they do. It has already been mentioned that in seeking to find the nature of X-rays attempts were made to produce diffraction effects with slits and that these were eventually successful. Such diffraction effects of X-rays are, however, of little practical value. The important X-ray diffraction phenomena are those observed with ruled gratings and with crystals. Grating effects are used in X-ray spectroscopy and crystal effects are used in spectroscopy as well as forming the basis of X-ray crystallography.