# Propositional variable

In mathematical logic, a **propositional variable** (also called a **sentential variable** or **sentential letter**) is an input variable (that can either be **true** or **false**) of a truth function. Propositional variables are the basic building-blocks of propositional formulas, used in propositional logic and higher-order logics.

Propositional variables with no object variables such as *x* and *y* attached to predicate letters such as P*x* and *x*R*y*, having instead individual constants *a*, *b*, ..attached to predicate letters are propositional constants P*a*, *a*R*b*. These propositional constants are atomic propositions, not containing propositional operators.

Propositional logic is sometimes called zeroth-order logic due to not considering the internal structure in contrast with first-order logic which analyzes the internal structure of the atomic sentences.