The Cavalier King Charles Spaniel's story starts, unsurprisingly, with a king. Small spaniels were fixtures of the English court for centuries, and King Charles II was so devoted to his that he reportedly allowed them everywhere — including, legend has it, the halls of Parliament.
The breed as we know it today, though, is younger than that legend suggests. By the 1800s, breeding trends favored a flatter face and domed skull, closer to what we'd now call a King Charles Spaniel (the English Toy Spaniel). The longer-muzzled, flatter-domed look of the original court spaniels nearly disappeared.
In the 1920s, an American breeder living in England, Roswell Eldridge, offered a standing prize for spaniels that matched the older, "Old World" type shown in historic portraits. The dogs that met his standard became the foundation of the modern Cavalier King Charles Spaniel, officially recognized as a distinct breed in the UK in 1945 and by the American Kennel Club in 1995.
So the Cavalier sitting on your couch is, in a real sense, a restoration project — bred back toward a look and temperament that royalty loved centuries ago, and that owners have loved ever since.