An estimated four percent of all people living in England and Wales can claim some royal ancestry and 13,890 of them will be married this year, according to Ancestry.com.
Kate Middleton, who weds Prince William on Friday, can trace her family tree back to Edward III of Windsor, perhaps a distant relative of her future husband. He reigned for 50 years in the 1300s during the Black Death and was known for his moods and his clemency.
In celebration of the royal wedding, ancestry.com is opening up its trove of British marriage records -- 96 million in all from 1837 until 2005 -- so Americans can search up until April 30 for free for their royal roots.
That also includes the "royal collection," a half-million nuptials of royalty, peerage, nobility and landed gentry.
Watch a special "20/20" Thursday at 8 p.m. ET for a behind-the-scenes look at the life that awaits Kate Middleton, and join us again at 4 a.m. Friday for ABC News' live coverage of the royal wedding with Diane Sawyer and Barbara Walters.
Anastasia Harman, lead family genealogist for ancestry.com, said she cannot "confirm or deny" that William and Kate have a common ancestor.
"Prince William is of the House of Windsor, but his family changed during World War I from the German name Saxe-Coburg and Gotha to the English Windsor by William's second great-grandfather, King George V, by proclamation," she said.
King Edward III belonged to House of Plantagenet and hailed from the town of Windsor.
"Back then, people would be called by their first name and place they were from or born -- "King Edward of Windsor" or "Robin of Locksley," and so on," she said.
Genealogy can uncover some striking family resemblances.
Robert Pattinson of the "Twilight" films is a distant cousin of Vlad the Impaler, the original Dracula. Emma Watson, the actress who played Hermione in the wizard movie series, "Harry Potter" movies, is related to a 15th century woman who practiced witchcraft, according to Harman.
Searching the family tree can also reveal strange political bedfellows.
President Barack Obama, a Democrat, is related to conservative radio host Rush Limbaugh. He also shares family history with Berkshire Hathaway CEO Warren Buffett, Tea Party favorite Sarah Palin and movie actor Brad Pitt.
On a quick scan of the royal site, this reporter found Sir Stuart Alexander Donaldson -- perhaps a distant relative -- who was born in London in 1812, the third son of an esquire who was later knighted.
No money or title in the family?
"You could be related to Kate, whose mother's name was Goldsmith," said Harman. "She has common and not-so-common roots."
Other Middleton family names include Meadows, Fairfax, Tanfield and Gascoigne, one with French lineage. Her connection to King Edward III comes through the Harrison line.
The website also provides an interactive family tree for both Kate and William. "You can go back in time and see some of the illustrious family connections," she said.
The British began national records for births, deaths and marriages in 1837," said Harman. "Before then, they were kept by the Church of England."
In the United States, reliable records depended upon the state in which you lived. "The U.S. didn't have 50 states in 1837, and some states didn't start until the early 1900s. Some weren't even states then," she said.