Geoffrey ChaucerThe Knight's Tale" found in The Canterbury Tales, is the story of two knights from Thebes who fall in love with the same woman, a princess of Athens named Emily. Since the two knights have apparently sworn to support each other in everything, each one's love for Emily does not go over well.
The name of the game in "The Knight's Tale" is chivalry, a system of rituals, duties, and behaviors a knight was supposed to follow if he wished to behave with honor. The rules of chivalry included things like always keeping your promises, defending the helpless, and remaining loyal to your lord and fellow knights no matter what. Think King Arthur and you're on the right track.
"The Knight's Tale" is also concerned with courtly love, which demanded the loyalty of the knight to just one person: his lady-love. Courtly love was actually a "system" of love, just as chivalry was a system of knightly behavior. That means there were rules...for love. The system got its start in the literature of the Aquitaine region in France, where troubadours sang ballads about the often secret and illicit love of knights for noblewomen (scandalous!).
The woman in a courtly love story is placed on a pedestal: she is totally perfect in every way, and the knight practically worships her. In fact, his love for her makes the knight stronger and more honorable. The rules of courtly love were even written down in a treatise by a 12th-century French courtier, Andreas Capellanus, in a work called De Amore, although literary types disagree on whether or not this work is meant to be serious or just a way to make fun of the courtly love tradition.
In any case, we have these two codes of behavior: chivalry and courtly love; and in "The Knight's Tale" we get to see what happens when the two codes clash.
Palamon and Arcite are sworn brothers. As brother knights, they should be willing to do anything to protect one another. But when they both fall into (courtly) love with Emily, they have to be willing to do anything to win her, which includes breaking their promise to one another. Or does it? That's the question "The Knight's Tale" wants you to think about.
Brought in to solve the conflict, we have the almost impossibly noble Duke Theseus. He represents another of the tale's major themes: order. What happens when two systems come into conflict? Answer: you need someone smart and powerful like Duke Theseus to figure out what to do.
Theseus's calming, powerful presence in the tale represents authority overcoming the forces of chaos. It reveals this tale's origins in the aristocratic genre of courtly romance, which portrays the aristocracy as a force for good in an otherwise dark, crazy, and scary world.