Crystals are often idealized as being "perfectly" periodic. In that ideal case, the atoms are positioned on a perfect lattice, the electron density is perfectly periodic, and the Fourier transform "F"(q) is zero except when q belongs to the reciprocal lattice (the so-called "Bragg peaks"). In reality, however, crystals are not perfectly periodic; atoms vibrate about their mean position, and there may be disorder of various types, such as mosaicity, dislocations, various point defects, and heterogeneity in the conformation of crystallized molecules. Therefore, the Bragg peaks have a finite width and there may be significant "diffuse scattering", a continuum of scattered X-rays that fall between the Bragg peaks.