As Joseph Ellis wrote in his book Founding Brothers: The Revolutionary Generation, John Adams and Thomas Jefferson could be considered “the odd couple of the American Revolution.” They first met as delegates to the Continental Congress in 1775; the following year, Adams would personally select Jefferson to draft the Declaration of Independence. Profoundly different in physical appearance and demeanor—Jefferson was tall, elegant and philosophical, while Adams was short, stout and prone to vivid outbursts of emotion—the two men nevertheless became close friends.

The friendship grew stronger in the 1780s, when Adams and Jefferson served diplomatic missions to Europe. While living in England and France, both Adams and his wife, Abigail, consoled Jefferson after the loss of his wife, Martha, and grew to consider him almost a part of the family.

Things got more complicated, however, when both men returned to the United States, and the heated debate over the new nation’s government. As secretary of state in George Washington’s cabinet, Jefferson was driven by a fear of a powerful central authority and gravitated toward the new Republican Party. Adams, who as vice president was largely marginalized in Washington’s administration, favored a strong central government to ensure the new nation’s survival, and aligned himself with the Federalist Party.

Jefferson’s enduring support for the French Revolution—even after the execution of King Louis XVI and the dawn of the Reign of Terror—further soured his friendship with Adams. His anger over Washington’s policy of neutrality led Jefferson to resign from the cabinet at the end of 1793 and withdraw to Monticello, his Virginia estate. It was during this period, according to Mark Silk, that Adams took the opportunity to gossip about his former friend in letters to his sons Charles and John Quincy.

Silk, a professor of religion and director of the Leonard E. Greenberg Center for the Study of Religion in Public Life at Trinity College, writes in Smithsonian about two letters written by Adams in January 1794, soon after Jefferson’s return to Monticello. In the first, addressed to Charles, Adams wrote of Jefferson’s supposed retirement from public life, saying that when Washington died or resigned, his former friend expected to be “invited from his conversations with Egeria in the Groves” to take control of the government. In a similar reference the following day, he wrote to John Quincy of Jefferson being “summoned from the familiar society of Egeria” to take the reins of power.

At the time, Silk argues, “conversation” was a euphemism for sexual intercourse, while “familiar” was a synonym for “intimate.” He believes the references to Egeria were Adams’ sly way of referring to Sally Hemings, the slave woman whose longstanding relationship with Jefferson produced (according to DNA evidence) at least one and probably six children between 1790 and 1808. In the early mythology of early Roman history (as chronicled by Livy and Plutarch), Egeria was a divine nymph or goddess who became the lover of Numa, a man chosen by Roman senators as their king after the death of Romulus, Rome’s founder.

Numa was a widower (like Jefferson) and the more philosophical and intellectual successor to a military hero. Silk believes the classical reference, though overlooked by later historians and biographers, would have been clear at the time. A French writer had published a popular novel about Numa in 1786—a year before Hemings, a half-sister of Jefferson’s late wife, accompanied Jefferson’s younger daughter, Mary, to Paris, where Jefferson was serving as a minister. Adams would certainly have known about the young, attractive slave girl in Jefferson’s household, as she and Mary stayed with the Adamses in London after their transatlantic voyage. If Silk’s theory is correct, it would suggest that the rumors of Jefferson’s liaison with Hemings would have been circulating—at least among the political elite—by 1794, long before they were first reported in the press.

True to Adams’ predictions, Jefferson wasted no time in seclusion, emerging after Washington stepped down in 1796 to run for president—against his former friend. After Adams won a narrow victory, he approached Jefferson with the idea of joining forces in a sort of bipartisan administration, despite the opposition of his Federalist cabinet. Jefferson declined, deciding it would not serve him well as leader of the Republican opposition to be drawn into the policy-making process of the administration. His refusal caused a definitive break between the two men during Adams’ presidency. Jefferson and James Madison formed a powerful Republican alliance, while Adams largely ignored his cabinet and relied on Abigail and his family for advice.

The 1800 election still stands as one of the nastiest in history. Jefferson’s supporters accused Adams of having a “hideous hermaphroditical character,” while Adams’ camp called Jefferson “a mean-spirited, low-lived fellow.” Jefferson hired a sleazy journalist, James Callendar, to smear Adams in the press, including the (false) story that he wanted to start a war with France. On the day of Jefferson’s inauguration, Adams took the early stagecoach out of Washington to rejoin Abigail in Quincy, and was not present during the ceremony. They would not exchange another word for 12 years.

Meanwhile, after serving jail time under the Sedition Act for his libel of Adams, Callendar demanded a government post in return for his service. When Jefferson failed to come through, Callendar uncovered and published the first public claims about Jefferson and his slave mistress, dubbed “Dusky Sally,” in a series of newspaper articles in 1801. No denial came from the White House, and the story would follow Jefferson for the rest of his career.

A mutual friend and fellow signer of the Declaration of Independence, Benjamin Rush, deserves credit for reigniting the Adams-Jefferson friendship. Around 1809, as Ellis related in “Founding Brothers,” Rush was simultaneously writing to Adams and Jefferson, suggesting to each man that the other was eager to resume the friendship. Rush told Adams he had dreamed about Adams writing to Jefferson, after which the two giants would renew their friendship through a correspondence. They would discuss their past disputes, and share their profound musings on the meaning of American independence. After that, in Rush’s dream, the two men “sunk into the grave nearly at the same time, full of years and rich in the gratitude and praises of their country.”

Amazingly, it played out almost just like that. On January 1, 1812, Adams sent a short note to Monticello. Over the next 14 years, he and Jefferson would exchange 158 letters, writing for posterity as much as for each other. Of the two, Adams wrote many more words, and was often the more confrontational and aggressive, while Jefferson maintained his characteristic philosophical calm. By the summer of 1813, the two men had regained a level of trust that allowed them to truly grapple with the two sides of the revolutionary legacy. That July, Adams wrote “You and I ought not to die before We have explained ourselves to each other.”

The famous correspondence touched on Adams’ vilification as a tyrant by Jefferson and his fellow Republicans, the unfairness of which Jefferson acknowledged. The two men also discussed the fallout of the French Revolution, the issue that had initially divided them back in the 1790s. In their later letters, Adams and Jefferson even anticipated the growing sectional tensions between North and South that would eventually result in the Civil War. However, true to the revolutionary generation’s shameful silence on the issue of slavery, they rarely touched on the taboo topic itself.

Even after Adams’ beloved Abigail died in 1818, and the two revolutionary patriarchs grew old and infirm, they continued writing to each other. “Crippled wrists and fingers make writing slow and laborious,” Jefferson wrote in 1823. “But while writing to you, I lose the sense of these things, in the recollection of ancient times, when youth and health made happiness out of every thing.”

Jefferson, who was suffering from an intestinal disorder, fell into a coma on the evening of July 3, 1826. He lingered in semi-consciousness until just after noon on the next day. That same morning, Adams collapsed in his reading chair, lapsing into unconsciousness around the same time Jefferson died. He woke up briefly around 5:30 that evening, and uttered his last words (either “Thomas Jefferson survives” or “Thomas Jefferson still lives,” according to different accounts) before dying. It was July 4, 1826—the 50th anniversary of Independence Day.