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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.
The internal structure of propositional variables contains [[predicate symbol|predicate letters]] such as P and Q, in association with [[free variable|free or bound]] individual variables (e.g., x, ''y''), individual constants such as ''a'' and ''b'' ([[singular term]]s from a [[___domain of discourse]] D), ultimately taking a form such as P''a'', ''a''R''b''.(or with parenthesis, <math>P(11)</math> and <math>R(1, 3)</math>).<ref>{{Cite web|date=2015-06-24|title=Mathematics {{!}} Predicates and Quantifiers {{!}} Set 1|url=https://www.geeksforgeeks.org/mathematic-logic-predicates-quantifiers/|access-date=2020-08-20|website=GeeksforGeeks|language=en-US}}</ref>
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.
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