On October 7, the New-York Historical Society opens the largest exhibit in its history, on the slave trade in New York. In the New York of the Colonial era, slaves made up 20% of the city’s population – between 5 and 10 times the proportion in Boston and Philadelphia.

“The exhibit shows how the labor of slaves was critical to the development of New York as a commercial center, from the very beginning through emancipation,” the president of the New-York Historical Society, Louise Mirrer, said.

In Manhattan, slaves built the forts and roads, including Broadway (ironically, the original road to Harlem), the Bowery, and the wall of Wall Street.

“Later on, when the hard labor was done, it just became fashionable for people to keep slaves at home,” Ms. Mirrer said.

Ownership of slaves was not limited to the wealthy, as the ledger of a white laborer, Henry Cormier, shows. For his work on the city’s docks, he billed his own time, as well as that of his slaves.

Society officials expect the exhibit, scheduled to run through March, to draw record numbers of visitors. But starting this month and throughout the show, the society is holding intimate gatherings for people of influence.

About 30 guests attended a dinner party last week to discuss slavery in New York, including the historians Kenneth Jackson, Henry Louis Gates Jr., Eric Foner, Ira Berlin, and Richard Rabinowitz; the lawyer Gordon Davis; the chief of policy at the Partnership for New York City, Ernest Tollerson; the writer Susan Fales-Hill; the president of the Association of Black Foundation Executives, Kenneth Austin; the director of the Schomberg Center for Research in Black Culture, Howard Dobson; the president of the Community Service Society, David Jones, and the secretary of Columbia University, Keith Walton.

“This is great. It’s like being in a graduate seminar,” Mr. Gates, a society board member, said as he welcomed questions.

Mr. Gates has just returned from Angola, where he was working on a PBS documentary on the genealogy of several prominent African-Americans, including the television star Oprah Winfrey, the surgical pioneer Benjamin Carson, and Bishop T.D. Jakes. With the help of DNA samples, the film traces its subjects’ family trees as far back as their African tribe.

“Let me tell you, Oprah is no Zulu,” Mr. Gates said.

Mr. Gates, a star professor at Harvard, examined his own genealogy as well. He learned that on his father’s mother’s side he descends from a New York Dutchman, Alexander Van Meter.

“Now I know why I love New York so much,” Mr. Gates said.