"Le onze Septembre Francais" declared the banner headline in France's Le Monde newspaper. "A French September 11th."
But now France also has 1/11, another set of numbers etched in the annals of the terrible history of terrorism and stirring stories of people who fight back.
Nearly four million took to the streets and squares across France on Sunday 11 January for a Unity march in the wake of three days of terror that left 17 dead, and a nation saddened, stunned, and - it is hoped - strengthened in a new solidarity.
Never have so many French citizens turned out in such large numbers since the liberation of Paris from Nazi Germany in 1944.
"I'm proud of my country," reflects French journalist Nicolas Henin. "People often join protests for more jobs or higher salaries, but this wasn't for something immediate. It was for France's future."
More than a decade ago, in the wake of the shocking attacks in New York and Washington on 11 September, the slogan that sounded from many capitals was: "We're standing shoulder to shoulder."
Now there is a new version of the same refrain in the most pronounced show of solidarity since then: "Le monde se leve" (The world rises) was the Sunday morning headline in Le Journal du Dimanche.
The morning after, one word was splashed across many front pages in France - "Debout" (Standing). "The world stands with France" was the top line of the International New York Times.
Nearly 50 world leaders cleared their diaries to literally stand shoulder to shoulder again.
They marched, arms linked, along part of the route to be, and be seen to be, part of a French story of pain and resolve.
Now, more than ever, this battle is recognised as a global fight against rising extremism. But some leaders had other reasons too.
The media watchdog Reporters Without Borders condemned the presence of "predators… the leaders from countries where journalists and bloggers are systematically persecuted".
"It's shocking to see some of the leaders here," laments journalist Nicolas Henin.
But, as we stand in the midst of the densely packed, diverse, cheering, flag-waving crowd at the main gathering place of La Place de la Republique, he speaks of a "feel-good day" for millions of people who came from across France and beyond.
"I had to come too," he remarks.
It was in this very place that smaller but still significant crowds had gathered every month, for 10 months, to call for his release, and for the three other French journalists who had been held hostage by the Islamic State group in Syria.
Sunday's call for unity came in the wake of the attack on the satirical magazine Charlie Hebdo and the murder of  four legendary French cartoonists who have long wielded the sharpest of pens.
Theirs is a history of France, from the street protests of 1968, to the civil conflict in Algeria in the 1990s, to the worst terrorist attack in half a century that cost them their lives.
By Friday, a series of attacks had also claimed the lives of three policemen, four members of the Jewish community, a cleaner and a bodyguard at the magazine's offices who got caught in the line of fire.
"Terrorism is about attacking symbols," remarks the Paris-based veteran US correspondent Christopher Dickey, Foreign Editor for The Daily Beast.
"This is the real similarity between the 9/11 attacks of New York and Washington and what happened in France," he said.
Both attacks struck at the very heart of a society and its sense of self.
The dramatic collision with the tall gleaming towers of New York, symbols of US economic enterprise and ambition, was in France a bloody strike against a very French freedom of expression: the right to satirise, to provoke, to push people and propaganda from pedestals of piety and power.
In the tributes that flowed from the pencils and pens of cartoonists worldwide, Ruben Oppenheimer sketched the twin towers as two tall sharp pencils, with an airplane headed their way.
In the gruesome mathematics of murder, the attacks are decidedly different.
The al-Qaeda attacks in the US killed some 3,000 people in New York and Washington by turning aircrafts into mammoth, metal suicide bombers.
Seventeen people died in three days of terror in France unleashed by three gunmen wielding assault rifles and rocket launchers.
But day in, day out, since Wednesday's shooting at Charlie Hebdo, people I have met in the streets and squares have invoked that fateful date.
"It is our 9/11" one man declared in the Place de la Republique just hours after the Paris shooting. "It's an attack on our Republic. It's an attack on our humour."
"In very French fashion, people are exclaiming Voltaire has been murdered," said one of France's leading journalists Christine Ockrent in a reference to the French philosopher and satirist of the 18th century, who deployed his sharp wit against the intolerance, religious dogma, and politics of his day.
"But our democracies must face the fact that it is no longer 9/11," she reflects. "A far different war is taking shape in so many countries."
9/11 was a sudden jolt from outside. France's moment emerged from within, as it has in other Western capitals from London to Ottawa to Sydney.
It shocked but it did not surprise.
In France, a country with the largest Muslim and Jewish populations of Europe, there have been growing strains and recurrent violence.
In a nation where many immigrants adhere to the Muslim faith, there has been growing anger and estrangement fuelled by economic disparities, and a polarising debate over values and freedoms.
The burning question now is what percentage of France's Muslim population shares the extremist ideology of the three gunmen who carried out these latest attacks and created a massive security operation on an unprecedented scale.
At Sunday's rally I met countless Muslims from across France and beyond, who condemned extremism and expressed fears of a possible backlash.
"I didn't like the Charlie Hebdo cartoons," said one man of the irreverent drawings which some saw as insensitive and sensational.
"But that's the cartoonists' business, not mine," he added. "They never killed anyone."
9/11 has another echo in France where many believe it was not the event itself but the reaction to it that has helped fuel mounting militancy.
"Its reaction was the Patriot Act, the Iraq war, Guantanamo Bay, repression, aggression and paranoia," wrote prominent French analyst Dominique Moise who is a senior adviser at the French Institute of International Affairs.
"The modern history of the United States falls into two periods, "before 9/11" and "after 9/11," he said.
"In the history of France, there will be also be a before… and after."
Now France has its 1/11, a day when the people of France made a statement, and made history.
Now its politicians and intelligence services have an even greater task of reconciling much vaunted freedoms with greater vigilance.
Will it still be fractious politics as usual, with the far-right, anti-immigration National Front (FN) party of Marine Le Pen gaining in popularity?
Will there still be what Jewish leaders say is about three anti-Semitic acts a day? Other communities complain of everyday racism.
"We need to stand up for more than a day," remarked one man who told me he came to La Place de La Republique because he felt "lost" after the attacks.
"We need to stand together for a long time to come."
In France, many reached for the phrase recognised by all - 9/11.