Physics and Astronomy Classification Scheme

"The Physics and Astronomy Classification Scheme® (PACS) was developed by the American Institute of Physics (AIP) and has been used in Physical Review since 1975 to identify fields and sub-fields of physics. It is used in a variety of ways, for example, in the online journals as a tool in searching for articles by subject. PACS is arranged hierarchically, by subdivision of the whole spectrum of subject matter in physics- and astronomy-related sciences into segments and then repeating the process of subdivision down to four levels. The latest edition of PACS is the 2010 edition."