Paris: "Copenhagen". The mere mention of the Danish capital's name can send a chill down the spine of even the toughest climate negotiator.
It was there in December 2009 that high hopes for a legal pact to curb climate-harming greenhouse gases came crashing down as diplomacy foundered in extra time.
Now, six years later, 195 nations will try again, this time in Paris.
Much has changed in the climate arena since 2009, and observers say there is reason to be hopeful that negotiators may finally seal some sort of deal.
"The world has learned some valuable lessons from the experience in Copenhagen," former US vice president turned climate activist Al Gore told AFP.
A key difference is that heads of state and government, who swooped in at the end of the 2009 summit, have been invited to attend only the first day in Paris.
When leaders failed to reach consensus six years ago, a handful among them -- representing key players such as the United States, European Union, Japan, China, India and Brazil -- huddled together to thrash out a face-saving "accord".
Instead of ratifying it, shell-shocked delegates simply "took note" of the non-binding document -- an event French President Francois Hollande remembered this week as "an immense failure".
"It was the worst moment in my life," Maldives negotiator Amjad Abdulla told AFP.
"We all worked very hard day and night. And at the very last minute we were told there is a text being negotiated by the heads of state," he recalled.
"You can't just... Pick... Some people and say this is the draft, take it or leave it. That's what happened in Copenhagen."
To avoid a repeat, summit host France has opted to leave heads of state out of the nitty-gritty haggling over text.
Instead, they will give back-to-back speeches on the first day of the November 30-December 11 marathon, seeking to imbue it with a sense of mission.
Bureaucrats who have been hammering out a blueprint for the last six years will take a final stab at drafting, and then leave it to ministers to seal the deal.
Another change is that delegates to the 21st annual Conference of Parties (COP 21) in Paris will work with a much slimmer draft, weighing in at 55 pages.
"This is the first time before a COP that we have such a small text. Before Copenhagen we had 300" pages, France's climate negotiator Laurence Tubiana said.