If kate middleton is not royal, how is it possible for her to trace her family history back so long?

Their common ancestors are Sir Thomas Fairfax and his wife Agnes who died almost 5 centuries ago. The bloodline that leads to Prince William is a non royal bloodline that comes to him via his mother. Sir Thomas Fairfax (died 1520) m. Agnes Gascoigne .Nicholas Fairfax m. Jane Palmes 1 .Margaret Fairfax m. Sir William Belasyse 2 .Sir Henry Belasyse 1 Bt m. Ursula Fairfax 3 .Thomas Belasyse 1 V Fauconberge m. Barbara Cholmondeley 4 .John Belasyse 1 B Belasyse m. Anne Paulet 5 .Barbara Belasyse m. Sir John Webb 3 Bt 6 .Mary Webb m. James Waldegrave 1 E Waldegrave 7 .James Waldegrave 2 E Waldegrave m. Maria Walpole 8 .Anne Horatia Waldegrave m. Hugh Seymour 9 .Horace Beauchamp Seymour m. Elizabeth Malet Palk 10 .Adelaide Horatia Elizabeth Seymour m. Frederick Spencer 4 E Spencer 11 .Charles Robert Spencer 6 E Spencer m. Margaret Baring 12 .Albert Edward John Spencer 7 E Spencer m. Cynthia Elinor Beatrix Hamilton 13 .Edward John Spencer 8 E Spencer m. Frances Ruth Burke Roche 14 .Diana Frances Spencer m. Charles Prince of Wales 15 .PRINCE WILLIAM of WALES (b. 1982) By the way, there are news articles circulating that they are 12th cousins once removed from a common ancestor: Sir Thomas Leighton married Elizabeth Knollys. That connection remains unproven, as it based on a book written in 1914, but has not been verified by any other means. As Sir Thomas Leighton is a more colorful (read nasty) historical figure it makes for a better news article Kate's male only line (Middleton) is only traceable back 6 generations to a "John Middleton" who was married in 1806. Only a relatively small part of her family tree is traceable back to the 16th century where a common ancestor with William has been found. If you count back 17 generations (i.e. fifteenth cousins) there should be 2^17 = 131, 072 people in your family tree. We know roughly 1,700 of those people in William's tree at that generation. I think they only know 2 people in Kate's family tree at that generation (their common ancestor). So in fact even with a royal we often know only 1% of some generations in their family tree. If it was possible to know more, the couple may be much closer than 15th cousins.

Being royal has nothing to do with it. If the records are available, anyone can trace it back - I'm British, not royal, and I know my ancestry at least back to the 17th century.

Plenty of non-royal people can trace their ancestries very far back, but you usually need a professional genealogist with access to the right archives and documents to help you. In Middleton's case, such people started working on her ancestry some time ago, because she'd achieved prominence as Prince William's girlfriend. They were curious, so they started doing the necessary research. Archival records contain lots of information about non-royals, since there have always been more of them than there have been royals. These archives exist in a wide variety of places. When I did research on my doctoral dissertation in the State Archives in Rome, Italy, I looked at old 16th-century church records that mentioned many people who did not hold royal or even aristocratic titles, but they'd been members of certain parishes, they'd donated money, they'd held funerals in the churches, and their children had been baptized in them. Church baptismal records are useful, because they mention the parents' names and the date of the baptism, which in earlier eras, was usually very shortly after the birth. My ancestry on my mother's side has been traced back to the early 17th century, and if I were to take the time on my next trip to the UK, I know I could trace it back further than that, because I know which archives to check and, as an academic, could get authorization to look at them. And I am not from a "special" family. Something else you may need is a person trained in the paleography -- the handwriting, orthography, and abbreviations -- of a given era and in the right language. The further back you go, the less likely it is that an amateur will have the right training, but professionals do have it. It's not easy even for a highly literate English speaker to read, say, 15th and 16th-century notarial documents in the English of those centuries. Various organizations, scholars, and staff at many archives have worked to transfer online (and translate) documents that may be of interest to people researching their family trees, so that they don't have to worry about archaic language and other details that they are not trained to read and understand.

15th cousins means they had a common ancestor 15 generations ago. At 20 years per generation, this is 300 years, so to about 1700, or maybe a bit earlier. I can trace my English ancestry back to someone living in the year 1400. So, it certainly can be done, especially for the "better" families - Landed Gentry, Yeomen, and so on. It is not always very easy, but it certainly is possible if the records for the particular church your ancestor went to still has it's records, and they are still in readable from. Many are.

I am not Royal.....just a very normal everyday person and I have researched my family history back to the 15th century......through cited records

Lots of people can trace their family history back. Many of them pretty far. have you ever heard of a genealogist? That's what they do.

I can trace my heritage to the eighteenth century when my ancestors came from France to America. So if I can do it anyone can.

I'm not Royal and have traced mine back to the 17th Century as well

No, Kate Middleton's father's family is not all that different from a lot of English families whose ancestry can be traced back to the 1700s. For this, however, genealogists have Henry VIII and the establishment of the Church of England to thank. As a byproduct of the Anglican Church's separation from the Roman Catholic Church in 1530, Parliament passed a law in 1537 that required ministers to record their parishes' baptisms, marriages, and burials. By 1598, ministers were yearly sending copies of their church registrars to an archdeacon or bishop. Thus, there was a uniform system of keeping all births, deaths, and marriages in England before civil registration began in 1837. Not everyone of English descent can trace all branches of his or her family tree back this far, but the 1537 law mandating the registration of all births, marriages, and deaths in local parish churches certainly accounts for the relative ease by which search engines of genealogical subscription services, such as Ancestry.com, can trace English ancestry back to the 16th century. Of course, it helps to have a relatively unusual surname. Surnames like Smith and Williams are much more difficult to trace. Miss Middleton probably had no idea that she was a 15th cousin to Prince William when the pair first meet, but upon her engagement to Prince William professional genealogists have traced her family tree. That's one of the perks of marrying the future King of England or running for President of the United States. P. S. -- Happy Holidays is correct, the shared ancestor is on the Spencer side of Willliam's family tree.