11th September 2021 marks 20 years since the moment the world stood still and witnessed the twin towers collapse.
In the lead-up to the anniversary, several documentaries featuring never-before-seen photos and videos of the attacks have been broadcast and people all over the world have relived the horror.
The release of a new Netflix movie set during the 9/11 attacks, which stars Batman's Michael Keaton, will also coincide with the 20th anniversary.
But one question which people still ask every year as the anniversary approaches is: why is 9/11 called '9/11'?
September 11 attacks: Why is 9/11 called '9/11'?
The most obvious answer to this question is the fact that the terrorist attack on the world trade centre in New York and the Pentagon in Washington, DC took place on 11th September 2001. In the US, that's September 11th which in short is 9/11.
But there's another theory behind the name - or rather the date - of the attacks.
Some believe that Osama Bin Laden and the 19 terrorists behind the plane hijackings specifically chose 11th September to target the World Trade Centre because 911 is the number for the emergency services in the US.
How many people died in the 9/11 attacks?
It's reported that a total of 2,977 people were killed in New York City, Washington, DC and outside of Shanksville, Pennsylvania (where the United Airlines flight 93 crashed).
Did anyone survive the twin towers collapse?
Out of the thousands of people trapped inside the twin towers at the time of the attacks, only 20 survived.
Among those, it's reported that 14 of the survivors were found inside the remnants of a stairwell at the center of the north tower.
One man lived to tell the tale after falling from the 22nd floor stairwell and regaining consciousness on top of the rubble on ground zero. And two police officers narrowly escaped their deaths after being trapped beneath the debris between the two towers.
All survivors were found on or near the top of the 3 billion pounds of rubble, which is probably why they made it out alive.
More than 400 of those who were killed in the attacks were rescue workers.
How many floors did the twin towers have?
The twin towers, which formed part of the World Trade Centre in New York City, had 94 floors.
The North Tower (1 World Trade Centre) was hit first and the hijacked plane crashed between the 93rd and 99th floors. Around 15 minutes later, another plane crashed into the South Tower (2 World Trade Centre) between floors 77 and 85.