Previous editions have probably been too chunky so this year I will do it in advent calendar form, with one question a day until the 24th. It might be all football/sporting trivia or it might be other stuff, I haven't decided yet. Answers in the thread, obvs. I reserve the right to give up at any time, although hopefully I should be bored/boring enough to carry on. First person to get the correct answer on a given day wins a spoilered photograph of a former Chelsea player of my choice.
Here's today's question:
Question 1
This achievement has been realised by 15 different clubs from eight countries.
Liverpool are the only English club to have done it (Man City have come the closest to emulating them).
Two French clubs have done it - PSG, and a much more unlikely side.
Bayern Munich are the only team to have done it three times.
Juventus were the first club to do it, but Real Madrid have never done it.
Steve McClaren once came within a single game of steering his club to this achievement.
What is it?