91 years ago today, the House of Windsor, Britain’s current royal house, was born.
The convoluted intermarriages of European royalty meant that from 1714 on, British monarchs were actually German. After Queen Anne, a Stuart, died without an heir, her cousin, a member of the House of Hanover, became George I. George III, our "foe" in the American Revolution, was German, his consort Queen Charlotte, didn’t speak any English at all. (Anybody seen the movie The Madness of King George?) For two centuries, the British Royal line was Hanoverian, and even Queen Victoria, arguably the most "British" monarch of all, was a member of that house. Her son, by virtue of her marriage to her first cousin Albert, was the first in the equally Germanic Saxe-Coburg-Gotha line. The next two generations carried the Saxe-Coburg-Gotha designation, but with the beginning of World War I in Europe, all things German were immediately suspect.
Interestingly, most of the monarchs of Europe were first or second cousins, once or twice removed; Victoria was called "The Grandmother of Europe," because she quite literally, was. Tsar Nicholas of Russia was her grandson; Victoria was the first known carrier of hemophilia, which crippled his rule and so famously killed his son, the young Tsarevich Alexi. /(Correction: Nicholas married Victoria's granddaughter, Alexandra. Tsarevich Alexi's illness came through his mother's line, not his father's. H/T to Pager in the comments.)/ So were Kaiser Wilhelm II of Germany and King Albert I of Belgium. During the early days of discussion preceding an outright declaration of war between the two empires, Tsar Nicholas and Kaiser Bill’s correspondence started with "Cousin Nicky" and "Cousin Willy".
On 19 June 1917, when King George V decreed that all subsequent issue of the British royal family would be known by the surname "Windsor," he did not choose the new last name blindly. Windsor Castle had for centuries served as one of the principal palaces of the royal family (its sanitary conditions were so awful that Victoria's consort Albert contracted typhoid there, which later killed him).
Queen Elizabeth, her Consort Prince Phillip, Prince Charles, and Princes William and Harry are all members of the House of Windsor. (If they weren’t Windsors, they’d be of the "House of Schleswig-Holstein-Sonderburg-Glücksburg," which is quite a mouthful, indeed.) The House of Windsor continues throughout the continent as well – Belgium, Denmark, The Netherlands, Norway, Spain, and Sweden all have Windsorian rulers.