Britain's Prince William and Kate, Duchess of Cambridge with their newborn baby son as they leave the Lindo wing at St Mary's Hospital in London. It is announced on Friday April 27, 2018, William and Kate's new son is named Louis Arthur Charles. Credit: John Stillwell | AP

William and Catherine, the Duke and Duchess of Cambridge, have named their third child Louis Arthur Charles.

Kensington Palace said on Friday that his official title will be His Royal Highness Prince Louis of Cambridge.

Louis is the name of the queen’s cousin, Lord Louis Mountbatten, who was killed by an IRA bomb in 1979. It’s also one of Prince William’s and Prince George’s middle names. Arthur is one of the middle names of William and King George VI, the queen’s beloved father. Charles is a tribute to that baby’s grandfather.

Prince Louis was born Monday morning.

He is fifth in line to the throne to succeed great-grandmother Queen Elizabeth II. Because of a 2013 act of Parliament, which removed preference for male heirs, he follows his sister, Princess Charlotte, in the line of succession.

William and Kate, as she is informally known, have previously picked traditional names with historical echoes for their children. George Alexander Louis, 4, may ultimately be Britain’s seventh King George. Charlotte Elizabeth Diana, 2, was named after her grandfather Prince Charles, her great-grandmother Elizabeth and her late grandmother Diana.

So it was unsurprising that conventional names were the favorites of the bookies before the latest announcement. Other names thought to be in the running were Arthur, James, Albert and Philip.

In Britain, where people can bet on nearly anything, bookmakers take in their biggest hauls for sporting events, such as soccer matches and horse races. But royal events can still attract large numbers of people placing small bets.

“The average bet for the royal baby name has been about one pound. The vast majority of people are having some fun. They are people like me, I have a son called Harry, so I put a pound on that,” said Rupert Adams, a spokesman for the bookmaker William Hill.

He said betting spiked after the baby was born and that bookies take bets right up until the moment of an official announcement, unless they believe someone has inside information.

“With the date of the birth, for example, we left that up until 5:44 a.m., when somebody saw a convoy going into the hospital and called their friends and placed bets. We lost about 10,500 quid because of that,” he said.

The names of George and Charlotte were revealed when they were two days old. William’s name was announced after a week, and Charles’s wasn’t revealed publicly for a month.

In Britain, names must be registered within 42 days of the child’s birth.

Kate gave birth to her third child just after 11 a.m. on Monday. That evening, she emerged from St. Mary’s hospital in central London — with perfect hair, no less — and returned to their home at Kensington Palace, a short drive away.

But if many new parents couldn’t quite identify with Kate’s immaculate looks so shortly after giving birth, they could probably feel for William. Two days after the birth of his son, he was seen nodding off during an Anzac Day service at Westminster Abbey.

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