The Cavalier King Charles Spaniel is a small spaniel classed as a toy dog by The Kennel Club. It is one of the most popular breeds in the United Kingdom. Since 2000, it has grown in popularity in the United States. It is a smaller breed of spaniel, and Cavalier adults are often the same size as adolescent dogs of other spaniel breeds. It has a silky coat and commonly an undocked tail. The breed standard recognizes four colours: Blenheim, Tricolour (black/white/tan), Black and Tan, and Ruby. The breed is generally friendly, affectionate and good with both children and other animals. The King Charles changed drastically in the late 17th century, when it was interbred with flat-nosed breeds. Until the 1920s, the Cavalier shared the same history as the smaller King Charles Spaniel. Breeders attempted to recreate what they considered to be the original configuration of the breed, a dog resembling Charles II's King Charles Spaniel of the Restoration. Various health issues affect this particular breed, most notably mitral valve disease, which leads to heart failure. This appears in most Cavaliers at some point in their lives and is the most common cause of death. The breed may also suffer from syringomyelia, in which cavities are formed in the spinal cord, possibly associated with malformation of the skull that reduces the space available for the brain. Cavaliers are also affected by ear problems, a common health problem among spaniels of various types, and they can suffer from such other general maladies as hip dysplasia, which are common across many types of dog breeds.