additional information controls how the non-logical symbols can be used to form terms and formulas. For instance if f is a binary function symbol and c is a constant symbol, then f(x, c) is a term, but c(x, f) is not a term. Relation symbols cannot be used in terms, but they can be used to combine one or more (depending on the arity) terms into an atomic formula.

For example a signature could consist of a binary function symbol +, a constant symbol 0, and a binary relation symbol <.

Models

Structures over a signature, also known as models, provide formal semantics to a signature and the first-order language over it.

A structure over a signature consists of a set (known as the domain of discourse) together with interpretations of the non-logical symbols: Every constant symbol is interpreted by an element of and the interpretation of an -ary function symbol is an -ary function on that is, a function from the -fold cartesian product of the domain to the domain itself. Every -ary relation symbol is interpreted by an -ary relation on the domain; that is, by a subset of

An example of a structure over the signature mentioned above is the ordered group of integers. Its domain is the set of integers. The binary function symbol is interpreted by addition, the constant symbol 0 by the additive identity, and the binary relation symbol < by the relation less than.

Informal semantics

Outside a mathematical context, it is often more appropriate to work with more informal interpretations.

Descriptive signs

Rudolf Carnap introduced a terminology distinguishing between logical and non-logical symbols (which he called descriptive signs) of a formal system under a certain type of interpretation, defined by what they describe in the world.

A descriptive sign is defined as any symbol of a formal language which designates things or processes in the world, or properties or relations of things. This is in contrast to logical signs which do not designate any thing in the world of objects. The use of logical signs is determined by the logical rules of the language, whereas meaning is arbitrarily attached to descriptive signs when they are applied to a given domain of individuals.