Petra Spithost of the Netherlands now has 16,860 members in her online family tree. One of them is Pim, her 7-year-old son. Another is St. Arnulf of Metz, an influential 6th-century Frankish bishop (and, importantly, the patron saint of brewing). And a third is the reigning queen of Holland, Beatrix.

If the tree speaks true, Beatrix is Spithost's great-grandfather's third cousin's daughter-in-law's grandfather's 30th great-uncle's 28th great-granddaughter. Not exactly a close relation, sure, but can you do any better?

Spithost, a 39-year-old Web developer who lives with her husband and children in the Dutch province of Friesland, is the creator and chief custodian of the largest family tree on Geni.com. Geni is a newfangled genealogy Web site that launched in January and has amassed more than 500,000 registered users and a grove of family trees that contains more than 6 million names.

"We're trying to create a family tree of the whole world," said Geni's chief executive David Sacks, echoing the company's motto: Everyone's related.

Geni's basic mechanism is simple: To get started, you create a profile for yourself including your name, region, date of birth and a photograph — just like a profile on MySpace or Facebook. Then, beginning with your immediate family, you fill in as much of your tree as you can. Inviting relatives to join in can help provide missing links.

The service therefore depends on a kind of chain-letter dynamic, with every user ideally contributing several more contributors. At this viral clip, a tree can sprout hundreds of names in no time, fueled by nothing but the bit each person knows about his or her corner of the family.

One of Geni's quirks is that its family trees tend to grow sideways instead of vertically. You can blame all the dead people for that: Because they don't have e-mail addresses, the slackers can't pull their own weight, often leaving the past shrouded in mystery. And because we living folks are better at identifying living relatives, the trees get populated with a lot of live people.

But sideways-sprouting trees are not as interesting, as more of the links end up being by marriage rather than by blood. Once you get a few marriages away from your nuclear family — that is, over to the in-laws of in-laws' in-laws — the term "relative" becomes a relative term indeed.

That's when you need a self-proclaimed genealogy "addict" such as Spithost to start mining the databases, county registries, online encyclopedias and memories of the elderly for the names of the forgotten. She spends as many as 20 hours a week building out her mega-tree, and there's no end in sight. Spithost, who says she relishes the endless "puzzle" of tree expansion, personally has added 9,800 names and invited 143 relatives to join the project, several of whom went on to add thousands of names.

"It's like creating a history book of your own," she said via instant message. "How far can you go into the past? What did your family members do for a living? How many children did they have, how many stayed alive?"

Although collaborative family tree-building probably is the way of the future, what might be unprecedented about Geni is the way it harnesses the connective power of the Web to turn the family tree into a living family network, capable of bringing together members of a lineage that has spread across time and territory.

One such world tree is run by Victor Bello, a 25-year-old systems engineer from Los Angeles. Bello is the American-born son of an Italian father and a Venezuelan mother and has large extended families in both countries as well as in the U.S. Bello's Geni tree has 1,132 members, and he's directly related to 400 of them.

The actress Maria Bello, he found out, is no relation. But through Geni, Bello has been able to connect with relatives half a world away, including a large contingent in his ancestral town of Vibonati, Italy. "It's awesome — I've met cousins I didn't even know I had," he said, navigating to a profile of a striking female second cousin, Anna Maria Bello. "She even called me and we talked on the phone."

Did the almost-strangers have anything much to discuss?

"She's my cousin," he said, laughing. "You talk about everything!"

The Bellos take full advantage of Geni's multimedia and interactive features. A built-in Google Map shows clusters of colored dots wherever on Earth a Bello relative is to be found. In the "Photos" section of the site, Bellos have posted more than 50 albums of digital pictures, visible to everyone in the tree. The "Calendar" lists hundreds of birthdays and anniversaries ("I'll never miss one again," said Bello), and the "Discussion" forum lets relatives weigh in on pressing familial issues — such as when's a good date for the reunion?

Geni "does everything but make coffee," gushed Bello. A moment later, he remembered a question he had for his wife. "What's the name of the famous singer your brother's related to?" he yelled to her.

To be sure, from Spithost's relationship to the queen, to Bello's wife's brother's relation to the famous singer, it seems everyone has at least one publicly known person in the family tree. Screenwriter Jay Lavender ("The Break-Up"), 32, said by phone that he hoped to build his Geni tree up enough to confirm family lore.

His grandmother, he said, repeatedly had told him that his great-great-great-great-grandfather Hugh Barker was an ancestor of erstwhile "The Price Is Right" host Bob, making Lavender and Barker some sort of distant cousins.

"But you never know what's chatter and what's true," he said.

A moment later, Bello's wife remembered the name: Nana Caymmi of the legendary Caymmi family of Brazilian singers and songwriters. Bello looked up Nana's profile on the Geni tree. Yep, turned out Nana was his brother-in-law's father-in-law's ex-wife. Free tickets?