Crystalline materials are characterised by a regular atomic structure that repeats itself in all three dimensions. In other words the structure displays translational symmetry. Translational symmetry is illustrated in this image of the crystal structure of the mineral cordierite (Mg2Al4Si5O18) taken with a high resolution transmission electron microscope. The image is a projection through a very thin slice (~200 Å thick) of the atomic distribution. Black spots correspond to hollow channels through the structure and white spots correspond to regions of high electronic density, arranged around the channels in 6-fold rings (Scale: the distance between the black spots is ~ 9.7 Å). The structure consists of a simple group of atoms that repeats itself periodically in space. This periodicity can be revealed using the concept of a lattice.