The Italians have a saying: La legge immutabile dell'ex - the immutable law of the ex.

In football, it relates to a former player delivering a knockout blow to the team that sold or discarded him. And beneath the closed roof of Cardiff's Millennium Stadium, the Champions League final can add to that legend in four ways.

For a start, there is Zinedine "Zizou" Zidane. Once a Juventus player, he now picks the team for Real Madrid.

Juve's former patron, Gianni Agnelli, adored great players, and Zidane was one of the greats. But Juventus, also known as the Old Lady of Turin, knows that every man has a price.

Madrid met that price - €75 million (S$116 million) - in 2001, and Zidane became an ex-Juventus player.

He never won the Champions League wearing Juventus' zebra stripes. But he won it within 12 months of transferring to Madrid, scoring the sweetest left-footed volley we might ever see in the 2002 final against Bayer Leverkusen.

His contemporaries were not surprised by him then, but they are now.

Alessandro Del Piero, who shared a Juventus dressing room with Zidane, said on ESPN this week that he did not foresee Zizou becoming a coach.

Didier Deschamps, another Frenchman in the then Juve starting line-up, yes. Zidane, no.

"He's a quiet guy," Del Piero said. "But he knows what is inside the players, he loves Madrid, and the key was Ancelotti."

Carlo Ancelotti was the Juventus manager when Zidane played for them. And Ancelotti managed Real Madrid when Zidane took his first steps from youth team coach to Ancelotti's assistant at the Santiago Bernabeu.

When Ancelotti left, and the players rebelled against his successor Rafa Benitez, Zidane was the manager in waiting.

He stepped up to confound those who, from Turin to Madrid, regarded him as too quiet and too shy to be the boss man.

In 18 months, Zizou's record in the revolving chair as Real Madrid head coach reads: Matches 86, wins 64, draws 15, losses seven, goals for 241, goals against 90.

Zidane's Madrid won the Champions League in his first season, won LaLiga in his second, and are a game away from becoming the first team in the Champions League era to win back-to-back finals.

How does he do it? Is it destiny? The answer is on the field where the famous white shirts of Real Madrid are filled today, as they have been throughout history, by the finest players money (and the lore of history) can buy.

Zidane has on his side Cristiano Ronaldo and Karim Benzema and, if fit for this final, the Cardiff-born Gareth Bale. He has Luka Modric and Toni Kroos, pass masters in midfield. And Marcelo, the flying Brazilian wing-back on the left.

And, should the well-stocked bench be called into play, he has Alvaro Morata. Talk about the ex. Morata was a child of Madrid, a youth team player and a first team player whom Real sold to Juventus, and then exercised the right to buy him back under a buy-back clause.

In effect, he is the insurance for when Ronaldo or Benzema are injured or rested from their nine-year double act.

Morata might very well be sold again by Madrid this month, but after him possibly returning in a substitute's role to deliver the immutable law of the ex.

Speaking of ex, Cristiano Ronaldo is immense, but not immeasurable. Former Real Madrid stars, before they were called galacticos, Alfredo di Stefano and Ferenc Puskas were world-class finishers in the fabulous Madrid XI that won the first five tournaments of the old European Cup back in the 1950s.

So nothing is new, and all things are relative in Europe's historic competition.

And there is a Madrid presence on either side of today's contest.

Juventus will line up with two former Madridistas in Cardiff.

Sami Khedira, a 2014 World Cup winner, was Real Madrid's midfield workhorse from 2010-2015. Gonzalo Higuain scored more than 100 league goals for Madrid between 2007-2013.

Yet, the latter was surplus to the Benzema-Ronaldo tandem.

Higuain moved to Napoli for €40 million, and last year was sold to Juve for €90 million.

He never was a galactico. But a bull of a centre forward, hungry for goals since his days at River Plate, he worked his way to 91 goals in 146 games for Napoli, and 32 goals in 54 appearances so far with Juve where he forms a double spearhead with his fellow Argentinian, Paulo Dybala.

If the ex is indeed immutable, what odds would you give against Higuain striking against his former team-mates in this final?

Sergio Ramos trying to manhandle his old pal on one side. Zidane plotting for ways to deprive Gianluigi Buffon from getting his hands on the only trophy that has eluded this truly wonderful veteran keeper.

Buffon, like Zidane, has won every other prize this sport offers.

The keeper, now approaching 40 yet still dedicated and steeped in the desire to win this prize, stands behind the meanest defence in the world at this moment.

This will be Buffon's 842nd club game, his 622nd for Juve. He has conceded just three goals in 12 games during this year's Champions League run, but Madrid, with 32 goals in the same number of matches, is far and away the most prolific in attack.

Something has to give. And the odds are that it will come down to the immutable law of an ex, one way or the other.