Propositional calculus
Constants and schemata

Mathematicians sometimes distinguish between propositional constants, propositional variables, and schemata. Propositional constants represent some particular proposition, while propositional variables range over the set of all atomic propositions. Schemata, or schematic letters, however, range over all formulas. (Schematic letters are also called metavariables.) It is common to represent propositional constants by A, B, and C, propositional variables by P, Q, and R, and schematic letters are often Greek letters, most often φ, ψ, and χ.

However, some authors recognize only two "propositional constants" in their formal system: the special symbol ⊤ {\displaystyle \top }, called "truth", which always evaluates to True, and the special symbol ⊥ {\displaystyle \bot }, called "falsity", which always evaluates to False. Other authors also include these symbols, with the same meaning, but consider them to be "zero-place truth-functors", or equivalently, " nullary connectives".
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