The system of checks and balances included in the U.S. Constitution was strongly articulated and popularized by James Madison , especially in Federalist No. 51 , but the underlying idea was originally proposed by the French Enlightenment thinker Baron de Montesquieu in his work The Spirit of the Laws.

In most U.S. history and civics classes, the expected answer to the question “Who proposed the system of checks and balances that was included in the U.S. Constitution?” is Baron de Montesquieu , because his separation‑of‑powers theory directly inspired the framers when they designed the Constitution’s structure.

TL;DR: For a quiz-style question, the best answer is: Baron de Montesquieu.

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