Cristiano Ronaldo's move to Real Madrid back in 2009 is undoubtedly one of the biggest in football's history.

Not only was it for a world record fee but it ushered in a period of success that the club hadn't seen since the 1950s.

Ronaldo was already one of the world's best but he kicked things up another gear at the Santiago Bernabeu, regularly setting goal-scoring records and establishing himself as one of the all-time greats.

While he may only have won two La Liga titles in nine seasons at the club, he helped Real to an incredible four Champions League trophies in a five-year spell.

There's little doubt that, despite the then-record fee, Real got a bargain back in 2009 and one they'll do well to top.

But then, they clearly knew they'd be signing one of the greats - as proven by an incredible deal struck with Ronaldo back in 2008.

So sure were Real that they'd be signing a superstar that they made sure to guarantee the move a year before it happened.

That's according to then-Real president Ramon Calderon, who spoke to The Athletic.

“He extended his contract with United [in 2008] but he did it with the commitment to come to Real Madrid the year after – he said, ‘I am going.'", said Calderon.

"We knew for sure he would be coming, which is why we signed the commitment with a €30 million penalty for both parties.

“This meant that if we didn’t comply with the contract, we would have to pay Cristiano €30 million and vice-versa – he would have to pay us the same if he reneged.

"He was very happy at United. He told me many times that Ferguson was a father to him, how he gave him the chance to become the best player, how he loved the fans but he wanted to change and we started to talk.

“The penalty clause was a way to strengthen the commitment. Ferguson didn’t want to sell but he knew."

Making that kind of commitment is extraordinary - what if his form had tailed off the next year? What if he picked up a terrible injury? What if Real's plans changed?

They clearly knew what they were getting and were happy to go all in on it. Safe to say, they don't regret it.