The 1982 Classification System superceded the previous CR classification introduced in 1964. This revision, nine years after the scheme was implemented, is the most comprehensive of these changes. The system has proved to be robust and shows promise for validity well past the ten-year time span. The heart of the classification system is a tree, the easiest format in which to represent a hierarchical structure in a linear publication format. The classification tree is restricted to three levels in order that the tree be able to reflect accurately the essential structure of the discipline over an extended period.
Subject descriptors are thus a permanent part of the tree.
Subsequent revisions have been suggested and refereed by specialists in various areas of computer science, most noticeably the Computing Reviews editorial board. The first involves a numbered tree containing unnumbered subject descriptors, and the second involves a General Terms list. The tree consists of 11 first level nodes and one or two more numbered levels under each of these. The set of children of all first and second level nodes begins with a node General and ends with a node Miscellaneous . The second and third levels have numerical designations.