When UFC Hall of Famer Matt Hughes and former two weight class champ B.J. Penn meet for their rubber match on Saturday at UFC 123, it will be the culmination of an unlikely rivalry.

Dave Meltzer talks about the significance of this trilogy in MMA history:

The term "legendary trilogy" is a staple in boxing. Even people with little knowledge of the sport know about three-fight classics like Muhammad Ali vs. Joe Frazier, "Sugar" Ray Leonard vs. Roberto Duran, Erik Morales vs. Marco Antonio Barrera and Arturo Gatti vs. Mickey Ward.

Mixed martial arts doesn't yet have that kind of trilogy heritage, partly because when B.J. Penn and Matt Hughes go into the cage for the third time at UFC 123 on Saturday at the Palace in Auburn Hills, Mich., it will be only the sixth time in company history that a rivalry has spawned a third fight.

But before their first meeting, no one would have predicted that Hughes and Penn would create one of the sport's great rivalries.

Their early UFC careers ran on parallel tracks in different divisions. For three years, from 2001 to 2004, Hughes was the dominant welterweight in the UFC and Penn was the promotion's top lightweight. And never the twain shall meet, or so it was assumed at the time.

By 2004, Matt Hughes had fought in, and won, six UFC title fights. He'd beaten Carlos Newton (2x), Hayato Sakurai, Gil Castillo, Sean Sherk and Frank Trigg. He was universally regarded at the world's best welterweight. 

For his part, Penn had put together a more checkered career. He won his first three fights but then fell short in his attempt to take Jens Pulver's title at UFC 35. He then won two more fights before fighting Caol Uno to a draw in a fight that was supposed to crown a new lightweight champ. That fight killed the UFC's lightweight division for three years. Penn went off to Hawaii and beat Takanori Gomi.

Then suddenly it was announced that Penn would be challenging Matt Hughes for the welterweight title at UFC 46. The matchmaking was as unexpected as the outcome, a shocking upset win for Penn. Jonathan Snowden will be talking about that fight in detail later today. 

Their rematch came about through equally improbable circumstances. Penn had returned to the UFC after an acrimonious contract dispute only to lose his return fight to Georges St. Pierre via a controversial decision. GSP was primed for his second shot at Matt Hughes' title. But then he pulled up hurt and a creative Joe Silva offered B.J. Penn the title shot instead. I'll look at that fight tomorrow. 

Their third match came about through another unlikely series of coincidences. Hughes had lost his title to GSP and been driven out of title contention through further losses to GSP and Thiago Alves. Since then he's been ensconced on the "senior circuit" fighting high profile but mostly pointless matches against the likes of Matt Serra and Renzo Gracie.

Penn had returned to lightweight and gone on a great run, taking and holding the title while looking invincible at 155. Then he ran into Frankie Edgar and lost twice. 

Again Dana White and Joe Silva have turned lemons into lemonade by booking Penn and Hughes for the rubber match to conclude their trilogy.

Each man has a very different take on what this fight means. Hughessees the third match as the one that will decide who's the better man. As he told MMA Fighting earlier this year:

"The way I feel is that if I lose this fight with BJ, it's like losing three fights," said Hughes. "And if I win this fight, it's like wining three fights. It's the final of the trilogy. There's a lot on the line. Three fights are on the line for this 15 minutes BJ and I are going to do battle."

For his part, Penn thinks nothing he can do will erase Hughes' win in the second fight.

"I don't think it's exactly like Matt says. After the fight on Saturday, when I walk away the winner, I'll still know that he beat me the second fight," Penn said at the pre-fight press conference yesterday.

The fight might not mean anything for the UFC title picture, but it will mean a lot for the legacies of both men and should take its place along side their first two bouts as a historically significant fight as the conclusion of one of MMA's rare triologies.