As they walked up to greet the Archbishop of Canterbury, Justin Welby, the little one was seen making some amusing faces before giving him a formal greeting.
The duke and duchess have always emphasized their desire for their children to have normal lives, rather than "growing up behind palace walls". That decision was reportedly made some time ago and was not due to ill health.
Prince Louis was sleeping as he arrived at the ceremony, held by his mother.
Max Mumby/Indigo via Getty Images Prince Louis was born on April 23, 2018 in London, England.
Princess Charlotte, teach us your waving technique!
However, on exiting the chapel with her family - today marked the first time the young royals have been seen as a family of five - Charlotte couldn't help but give her adorable signature wave to her adoring public.
Louis´ other godparents are: Nicholas van Cutsem, whose family is close to the royals; William´s school pal Harry Aubrey-Fletcher; Laura Meade, who is married to William´s close friend James Meade; Kate´s schoolfriend Hannah Carter; and the duchess´s cousin Lucy Middleton.
I deserved penalty for Hamilton clash - Raikkonen
Ferrari lead the constructor standings with 287, ahead of Mercedes with 267. Ferrari's Sebastian Vettel celebrates winning the race.
Miss Middleton and Mr Pelly each read a lesson during the service, Kensington Palace said. Her woven Jane Taylor hat had a large flower and netting on one side.
The service for Prince Louis was attended by guests including Prince Charles and his wife Camilla, and newlyweds Prince Harry and Meghan.
Pippa Middleton arrives with her husband James Matthews.
They also chose two anthems for the choir to sing - This is the Day which the Lord hath Made, written for their wedding in 2011 by John Rutter, and Suo Gan, a traditional Welsh lullaby.
The tradition of this particular christening gown harkens back to Queen Victoria, who commissioned the garment for the royal baptism of her daughter Victoria in 1841; you can glimpse one of its earliest representations in a painting of the event from the era.
Welby baptised Louis by pouring holy water from the River Jordan over his head. He is fifth in line to the throne.
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