Mountbatten (78) was assassinated by the IRA, who detonated a bomb on his leisure boat on August 27, 1979.
It's a French name and the French had 18 kings named Louis: The French monarchy were historically very keen on the name Louis, and ended up with 18 monarchs bearing that name. The grandfather of Prince Philip was named Prince Louis of Battenberg.
The birth means that the Duke of York is now seventh in line to the throne (he was second at the time of his birth) and could marry without the Queen's permission.
Kate's new baby will be fifth in line to the throne after Charles, William, George and Charlotte.
Louis's names are nothing new: Louis itself is one of Prince William's middle names and one of big brother Prince George's middle names. Princess Charlotte eventually arrived on the bank holiday weekend, on Saturday 2 May. Hours later, she and Prince William publicly debuted their third child on the stairs of the Lindo Wing in London.More news: Suppliers could be squeezed by Asda Sainsbury's merger
The announcement may have caused Royal fever across the world, but not everyone was excited.
The list includes aunt Pippa Middleton (who is now expecting her first child with husband James Matthews), uncle James Middleton, grandma Carole Middleton, and his father's cousin, Princess Eugenie. A friend of the couple notes that Kate, 36, "is one of three siblings, and it's a good family size".
There are other royal Louises in the family, too.
The Daily Express recalls when Prince William and Kate Middleton met as students at St. Andrews University in Scotland.
The same logic applies to Kate, who married into the royal family, and to avoid confusion was given the title Her Royal Highness Kate, The Duchess Of Cambridge, Countess Of Strathearn and Lady Carrickfergus.