The Physics and Astronomy Classification Scheme® (PACS®) is a hierarchical subject classification scheme designed to classify and categorize the literature of physics and astronomy. PACS provides an essential tool for classification and efficient retrieval of literature in physics and astronomy; as such, PACS is used by AIP and other international publishers of journals in physics, astronomy, and related fields.PACS contains ten broad subject categories subdivided into narrower categories. The hierarchy includes mainly four levels of depth, with the narrowest term giving the most detailed characterization. However, beginning with the 2006 edition, a fifth-level hierarchy was introduced; subsequently, in this new edition, the fifth-level hierarchy is continued in sections that have undergone revision and will also be a part of future editions. PACS also includes detailed appendices for acoustics and geophysics, a nanoscale science and technology supplement, and a topical alphabetical index with corresponding PACS codes.。