That's where Mr. Heinegg, 60, comes in. In 1985, his mother-in-law, Katherine Kee Phillips, who was black, asked him to research her family tree. ''I had hoped to trace as many branches of her family back to slavery as possible,'' he said. Instead, he found that Mrs. Phillips and his wife, Rita, had white ancestors who were not slave masters, including a woman who started a family with John Kecatan, an African slave freed in 1666. The ladies were intrigued by his discoveries but not surprised, Mr. Heinegg said.

Curious about his findings, he began tracing free black families related to his wife by combing colonial court records, wills, deeds, free Negro registers, marriage bonds and military pension files. Many were dauntingly unindexed.

''Nobody has done anything like this,'' said Dr. Virginia Easley DeMarce, a historian and former president of the National Genealogical Society who works for the Office of Federal Acknowledgment, Department of the Interior, which decides who is an American Indian. ''Paul is the first person to identify families of color on such a broad scope,'' gathering material from entire states rather than just a county or two.

Dr. Berlin said, ''There were communities in 17th- and 18th-century America where blacks and whites, both free, of equal rank and shared experiences, were working together, living together, drinking and partying together, and inevitably sleeping together.''

Tracing those communities has not been easy. ''People of color are often not identified as such in early records,'' Mr. Heinegg said. ''For example, an individual might appear in deeds and court records and leave a will without ever mentioning his race.'' Sometimes a person's race can be discerned only by studying the tax assessed on nonwhites. If a man paid the tax on his wife but not himself, Mr. Heinegg said, it meant he was white but she was not.

An added challenge is that racial identity can mutate from free black to white in just a few generations. In my Archer ancestors' case, it was mixed marriages and a cross-country move: my great-great-grandfather Esquire Collins and his wife, Roxalana Archer, are listed as mulatto in an 1800's Tennessee census but show up as white on a later Arkansas census. ''You crossed over as early as you were able to,'' said Antonia Cottrell Martin, a genealogist in New York. Mixed-race families who had difficulty passing sometimes explained dark complexions as coming from an American Indian or Mediterranean ancestry. ''It's what people in the South used to call Carolina Portuguese,'' said Dr. DeMarce, who comes from a mixed-race background.

''Free African Americans of North Carolina,'' self-published by Mr. Heinegg in 1991, won an award from the North Carolina Genealogical Society. (The American Society of Genealogists gave a later edition the Donald Lines Jacobus Award for best work of genealogical scholarship.) But the book also stirred controversy. Some white members of the North Carolina group were upset with his findings and asked that the award be withdrawn, Mr. Heinegg said.