Who was the primary secondary source of the constitution?

The primary secondary source of the U.S. Constitution is often considered to be the Federalist Papers, a series of essays written by Alexander Hamilton, James Madison, and John Jay. These essays were published to promote the ratification of the Constitution and provide insight into the framers' intentions and the principles underlying the document. They serve as a crucial interpretive guide for understanding the Constitution's structure and the rationale behind its provisions.