A formal, mathematical description of access control that describes how security can be maintained by comparing the clearance of a subject with the classification of an object to determine authorization. In general, such mandatory access control systems enforce the rule that no one reads information from a higher level of classification, and no one writes information to a lower level of classification. The original work was by D. Bell and L. La Padula in the 1970s. For more information, read http://csrc.nist.gov/publications/history/bell76.pdf.