Kate Middleton isn't technically a princess — so why does her son's birth certificate say she is?

Prince Louis Arthur Charles of Cambridge’s birth certificate was released to the public on Tuesday — and there’s one detail that doesn’t make a lot of sense.

On the official document, the Duchess of Cambridge’s full name is written as Catherine Elizabeth Her Royal Highness The Duchess of Cambridge. And then, her occupation is listed as Princess of the United Kingdom.

Yet because only Queen Elizabeth’s children or grandchildren can be given such a royal title, it’s confusing as to why the Duchess of Cambridge (who, unlike Princess Diana, did not take on the title of princess) has been given one.

However, according to royal commentator Richard Fitzwilliams, there’s a simple explanation. “When Prince William married Catherine Middleton they were granted the title of Duke and Duchess of Cambridge,” he explains. “William is HRH Prince William, Duke of Cambridge and so Kate, as his wife, is HRH The Duchess of Cambridge. If you marry a prince you become a princess. Louis’ birth certificate would naturally reflect this.”

Following the death of his father, Prince Charles, William will become King and Kate will be given a new title too.

The duchess will most likely be Queen Consort, as Queen Elizabeth II's mother, also Queen Elizabeth, was known when her husband, King George VI, became the monarch.

When Prince Harry and Meghan Markle wed on May 19, the couple are set to take on the titles of the Duke and Duchess of Sussex.

See all the photos from the new prince's birthday!

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Duchess Kate gives birth to baby No. 3! All the photos from the big day
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Duchess Kate gives birth to baby No. 3! All the photos from the big day
Britain's Prince William, Duke of Cambridge (L) and Britain's Catherine, Duchess of Cambridge show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Daniel LEAL-OLIVAS / AFP) (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
LONDON, ENGLAND - APRIL 23: Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge depart the Lindo Wing with their newborn son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a son at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Karwai Tang/WireImage)
LONDON, ENGLAND - APRIL 23: Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge depart the Lindo Wing with their newborn son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a boy at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Samir Hussein/WireImage)
The Duke and Duchess of Cambridge and their newborn son outside the Lindo Wing at St Mary's Hospital in Paddington, London. (Photo by John Stillwell/PA Images via Getty Images)
LONDON, ENGLAND - APRIL 23: Catherine, Duchess of Cambridge departs the Lindo Wing with her newborn son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a boy at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Chris Jackson/Getty Images)
Britain's Catherine, Duchess of Cambridge holds her newly-born son as he's presented to the press outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Daniel LEAL-OLIVAS / AFP) (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. Photo by Daniel LEAL-OLIVAS / AFP 
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Daniel LEAL-OLIVAS / AFP) 
The newborn son of Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge as they depart the Lindo Wing at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a boy at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Chris Jackson/Getty Images)
The newborn son of Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge as they depart the Lindo Wing at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a boy at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Chris Jackson/Getty Images)
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by DANIEL LEAL-OLIVAS / AFP) 
The Duchess of Cambridge with her newborn son outside the Lindo Wing at St Mary's Hospital in Paddington, London. (Photo by John Stillwell/PA Images via Getty Images)
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Isabel INFANTES / AFP)
The Duchess of Cambridge and her newborn son outside the Lindo Wing at St Mary's Hospital in Paddington, London. (Photo by Dominic Lipinski/PA Images via Getty Images)
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge (R) show their newly-born son, their third child, to the media outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Ben STANSALL / AFP) 
Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge depart the Lindo Wing with their newborn son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a son at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Mike Marsland/Mike Marsland/ WireImage)
Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge depart the Lindo Wing with their newborn son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a son at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Mike Marsland/Mike Marsland/ WireImage)
Britain's Prince William arrives at the Lindo Wing of St Mary's Hospital with his children Prince George and Princess Charlotte after his wife Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018. (REUTERS/Henry Nicholls)
LONDON, ENGLAND - APRIL 23: Prince William, Duke of Cambridge arrives with Prince George and Princess Charlotte at the Lindo Wing after Catherine, Duchess of Cambridge gave birth to their son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a boy at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Steve Back/Getty Images)
Princess Charlotte of Cambridge (L) turns to wave at the media as she is led in with her brother Prince George of Cambridge (R) by their father Britain's Prince William, Duke of Cambridge, (C) at the Lindo Wing of St Mary's Hospital in central London, on April 23, 2018, to visit Catherine, Duchess of Cambridge, and their new-born brother, the Duke and Duchess's third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
LONDON, ENGLAND - APRIL 23: Prince William, Duke of Cambridge and Princess Charlotte arrive with Prince George at the Lindo Wing after the birth of his third child on April 23, 2018 in London, England. The Duchess of Cambridge gave birth to a boy at 11:01 BST, weighing 8lb 7oz. (Photo by Neil Mockford/GC Images)
LONDON, ENGLAND - APRIL 23: Prince William, Duke of Cambridge, Prince George and Princess Charlotte arrive at the Lindo Wing after the birth of his third child on April 23, 2018 in London, England. The Duchess of Cambridge gave birth to a boy at 11:01 BST, weighing 8lb 7oz. (Photo by Neil Mockford/GC Images)
Princess Charlotte of Cambridge (L) turns to wave at the media as she is led in with her brother Prince George of Cambridge (R) by their father Britain's Prince William, Duke of Cambridge, (C) at the Lindo Wing of St Mary's Hospital in central London, on April 23, 2018, to visit Catherine, Duchess of Cambridge, and their new-born brother, the Duke and Duchess's third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
LONDON, ENGLAND - APRIL 23: Prince William, Duke of Cambridge arrives with Prince George and Princess Charlotte at the Lindo Wing after Catherine, Duchess of Cambridge gave birth to their son at St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a boy at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Chris Jackson/Getty Images)
Britain's Prince William, Duke of Cambridge, gives a wave as he leaves the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018, after his wife Britain's Catherine, Duchess of Cambridge, gave birth to a son, their third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
Britain's Prince William, Duke of Cambridge, gives a wave as he leaves the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018, after his wife Britain's Catherine, Duchess of Cambridge, gave birth to a son, their third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
Britain's Prince William, Duke of Cambridge, gives a wave as he leaves the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018, after his wife Britain's Catherine, Duchess of Cambridge, gave birth to a son, their third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
Britain's Prince William, Duke of Cambridge, gives a wave as he leaves the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018, after his wife Britain's Catherine, Duchess of Cambridge, gave birth to a son, their third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
Britain's Prince William, Duke of Cambridge, gives a wave as he leaves the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018, after his wife Britain's Catherine, Duchess of Cambridge, gave birth to a son, their third child. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Isabel INFANTES / AFP) (Photo credit should read ISABEL INFANTES/AFP/Getty Images)
LONDON, ENGLAND - APRIL 23: Tourists and photographers take pictures of a notice placed on an easel in the forecourt of Buckingham Palace in London to formally announce the birth of a baby boy to the Duke and Duchess of Cambridge at the Lindo Wing of St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a son at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Stefan Rousseau - WPA Pool/Getty Images)
LONDON, ENGLAND - APRIL 23: A notice is placed on an easel in the forecourt of Buckingham Palace in London to formally announce the birth of a baby boy to the Duke and Duchess of Cambridge at the Lindo Wing of St Mary's Hospital on April 23, 2018 in London, England. The Duchess safely delivered a son at 11:01 am, weighing 8lbs 7oz, who will be fifth in line to the throne. (Photo by Stefan Rousseau - WPA Pool/Getty Images)
A Royal bulletin, announcing that Britain's Catherine, Duchess of Cambridge, the wife of Britain's Prince William, has given birth to a baby boy, stands on an easel on the forecourt of Buckingham Palace in London on April 23, 2018. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Stefan Rousseau / POOL / AFP) (Photo credit should read STEFAN ROUSSEAU/AFP/Getty Images)
Palace staff position a Royal bulletin, announcing that Britain's Catherine, Duchess of Cambridge, the wife of Britain's Prince William, has given birth to a baby boy, on an easel on the forecourt of Buckingham Palace in London on April 23, 2018. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Daniel SORABJI / AFP) (Photo credit should read DANIEL SORABJI/AFP/Getty Images)
A Royal bulletin, announcing that Britain's Catherine, Duchess of Cambridge, the wife of Britain's Prince William, has given birth to a baby boy, stands on an easel on the forecourt of Buckingham Palace in London on April 23, 2018. - Kate, the wife of Britain's Prince William, has given birth to a baby son, Kensington Palace announced Monday. 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 (1001 GMT),' the palace said in a statement. The baby boy weighs eight pounds and seven ounces (3.8 kilogrammes). (Photo by Stefan Rousseau / POOL / AFP) (Photo credit should read STEFAN ROUSSEAU/AFP/Getty Images)
Camera crews prepare outside the Lindo Wing at St Mary's Hospital in central London where Britain's Catherine, Duchess of Cambridge gave birth to a baby boy on April 23, 2018. (Photo by Daniel LEAL-OLIVAS / AFP) (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
Royal fans celebrate the news that Britain's Catherine, Duchess of Cambridge gave birth to a baby boy outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Isabel INFANTES / AFP) (Photo credit should read ISABEL INFANTES/AFP/Getty Images)
A town crier announces the news of the birth of a baby boy by Britain's Catherine, Duchess of Cambridge outside the Lindo Wing at St Mary's Hospital in central London, on April 23, 2018. (Photo by Ben STANSALL / AFP) (Photo credit should read BEN STANSALL/AFP/Getty Images)
A town crier outside the Lindo Wing at St Mary's Hospital in Paddington, London, after the news that the Duchess of Cambridge has given birth to a son. (Photo by Kirsty O'Connor/PA Images via Getty Images)
LONDON, ENGLAND - APRIL 23: A town crier announces that the Duchess of Cambridge has given birth to a baby boy at St Mary's Hospital on April 23, 2018 in London, England. The Duke and Duchess of Cambridge's third child was born this morning at 11:01, weighing 8lbs 7oz. (Photo by Jack Taylor/Getty Images)
LONDON, ENGLAND - APRIL 23: A town crier announces the birth of the new prince on the steps of the Lindo Wing on April 23, 2018 in London, England. (Photo by Neil Mockford/GC Images)
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