What is PACS?

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.

Depending on the topic, the most detailed PACS code may be found at the third, fourth, or fifth hierarchical levels. At these three levels, each PACS code consists of six alphanumeric characters divided into three pairs. The examples, in the table below, illustrate the structure and format of PACS codes for all levels of the scheme, using PACS codes where the hierarchy terminates at the third, fourth, and fifth levels:


to 3rd Level

to 4th Level

to 5th Level






Broadest category; there are 10 such codes from 00 to 90, in increments of 10.


04. General relativity and gravitation

32. Atomic properties and interactions with photons

91. Solid Earth physics

More specific category; up to 9 such codes under each 1st-level category.


04.65.+e Supergravity

32.10.−f Properties of atoms

91.25.−r Geomagnetism and paleomagnetism; geoelectricity

Fairly specific category; "−" or "+" as 5th character denotes presence or absence, respectively, of 4th level.



32.10.Hq Ionization potentials, electron affinities

91.25.F− Rock and mineral magnetism

Most specific category found in most of PACS; "−" or a lowercase letter as the 6th character denotes presence or absence, respectively, of 5th level.




91.25.fd Environmental magnetism

Most specific category found in PACS; the 5th character is the same as for the 4th-level code, but lowercase.

Note that the use of upper- and lowercase letters as the fifth character for fourth- and fifth-level codes, respectively, is a means to easily distinguish the level of a given code; the use of italics for the fifth level serves a similar purpose. However, case and font are not needed to determine uniqueness, i.e., there are no redundant codes.

How to Use PACS

In order to classify an article, the main topics presented in that article must be identified. The most specific PACS codes that describe the content of an article are then selected using the alphabetical index to PACS. The first code is reserved for the main topic of the paper. Select as many codes as are necessary to classify the paper; three to four codes are generally sufficient. For errata or related items, an additional code must be selected from the section, 99.10.-x Errata and other corrections.

Note for Publishers: Publishers are encouraged to contact AIP's Business Development department before using PACS in any publishing or related business activity. Both "PACS" and "Physics and Astronomy Classification Scheme" are registered trademarks of the American Institute of Physics.