Cain Velasquez and Junior dos Santos will meet for the third time on Saturday night in Houston, looking to settle the score in what promises to be the biggest heavyweight trilogy in UFC history. Velasquez (12-1) entered as the champ in the first fight -- as he does this weekend -- but dos Santos (16-2) knocked him out in the first round in November 2011. They two met again last December, but it was Velasquez who pummeled his foe for five rounds to reclaim the belt. Ahead of Saturday's pay-per-view UFC 166 main event, here's a look at the three biggest UFC trilogies.

Chuck Liddell vs. Randy Couture

The future UFC Hall of Famers first fought 10 years ago with the interim light heavyweight title up for grabs -- which was created when champion Tito Ortiz refused to defend the belt. Couture, previously the heavyweight champion, entered as the underdog but pulled off the upset by technical knockout in the third round. They met again two years later for Couture's undisputed crown, but this time Liddell needed just two minutes to knock out Couture. They met again the next year, and again it was Liddell by knockout, which sent Couture into his first retirement.

Frankie Edgar vs. Gray Maynard

Edgar-Maynard I was of little note at the time; just a 2008 meeting between two up-and-coming lightweights that Maynard won by clear decision. But by the time they fought three years later, Edgar reigned over the division as champ. Maynard looked like he would make short work of Edgar when he nearly finished him twice with strikes, but Edgar held on and fought back to earn a split draw. The final meeting in October 2011 went much the same way, but this time Edgar tagged his rival in the fourth and knocked Maynard out.

Georges St-Pierre vs. Matt Hughes

Today, St-Pierre is regarded as one of the best pound-for-pound fighters on the planet. But when he first challenged Hughes for the vacant welterweight title in October 2004, he was a highly touted prospect in over his head, affirmed by his armbar submission loss with 1 second left in Round 1. St-Pierre learned from the loss and returned two years later to knock out Hughes in Round 2. By the time St-Pierre submitted Hughes with an armbar of his own the next year, it was clear the torch had been passed. Fun fact: Since Hughes won the welterweight title 12 years ago, either he or "GSP" has held UFC gold for all but 18 months.

OUTSIDE THE OCTAGON

These trilogies were staged mostly in MMA promotions other than the UFC but were contested by elite fighters and filled with thrills.

Wanderlei Silva vs. Quinton "Rampage" Jackson

Silva stopped the first two with his knees in Pride FC in Japan, but "Rampage" got revenge at UFC 92 with a thunderous first-round knockout.

Gilbert Melendez vs. Josh Thomson

All three bouts for the Strikeforce lightweight championship went the distance, and all three were exciting affairs. Thomson won the first, but Melendez won the other two.

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