This International Standard specifies a family of logic languages designed for use in the representation and interchange of information and data among disparate computer systems. The following features are essential to the design of this International Standard: • Languages in the family have declarative semantics. It is possible to understand the meaning of expressions in these languages without appeal to an interpreter for manipulating those expressions. • Languages in the family are logically comprehensive — at its most general, they provide for the expression of arbitrary first-order logical sentences. • Interchange of information among heterogeneous computer systems. The following are within the scope of this International Standard: • representation of information in ontologies and knowledge bases; • specification of expressions that are the input or output of inference engines; • formal interpretations of the symbols in the language. The following are outside the scope of this International Standard: • the specification of proof theory or inference rules; • specification of translators between the notations of heterogeneous computer systems; • computer-based operational methods of providing relationships between symbols in the logical “universe of discourse” and individuals in the “real world”. This International Standard describes Common Logic’s syntax and semantics. It defines an abstract syntax and an associated model-theoretic semantics for a specific extension of firstorder logic. The intent is that the content of any system using first-order logic can be represented in this International Standard. The purpose is to facilitate interchange of first-order logic-based information between systems. Issues relating to computability using this International Standard (efficiency, optimization, etc.) are not addressed.