Low-energy electrons can diffract from many assemblies of atoms, including molecules [3.1], liquid surfaces [3.2], and otherwise disordered surfaces [3.3]. However, under the name LEED, this technique has been applied mainly to the study of single-crystal surfaces with a high degree of ordering of the surface atoms, especially as a tool for determining surface structures [3.4]. Therefore we shall be very much concerned with the detailed atomic structure of single-crystal surfaces. This chapter discusses the general properties and the nomenclature of surface structures and their connection with the LEED diffraction pattern that is observed experimentally. In particular, we explore the structural information that the diffraction pattern can provide without consideration of beam intensities
