The Bible: At the time he was writing, Chaucer's England was still dominated by the Catholic Church, even though much division and warring existed. Therefore, many of the metaphors are based on the Old and New Testaments of the Bible. Depending on which character is narrating, different metaphors are used. The Prioress focuses her metaphors on the Virgin Mary to strengthen her story and convictions, but the Wife of Bath uses Biblical references so that she can question the validity, intent, or interpretation of the Bible. Besides the Old and New Testaments, Certain Saints and post-biblical events are used as metaphors, for example the re-telling of the story of St. Cecelia.
Greek and Roman Mythology: The Mythology of Ancient Greece and Rome was known well in the time of Chaucer, although no one believed it as religion. Mythology was used more as a romantic and idealistic way to convey an emotion. For example, When the Knight used references to Greek stories, the people reading the story could take their knowledge of those Greek Stories and apply those emotions and ideas to the Knight's story, therefore making the Knight's story easier to understand.
Contemporary Authors of Chaucer's Time: Although these authors may not have been living during Chaucer's time, compared to Biblical and Greek stories, authors such as Dante and Bocaccio were much closer to Chaucer's time period. Chaucer, being a well-read and well-traveled man, used many of the newer works that he came across while travelling in his Canterbury Tales. These stories would have been less familiar to a reader in Chaucer's time, so he often explains them more fully, or refers back to the original source, so that his readers can find a clearer explanation of his metaphor, if they wish to.
Sexual Slang: The use of sexual metaphors and sexual slang is essential to The Canterbury Tales. Chaucer is often fearless in its use, which is another indicator of the times of freedom that he lived in. The language is different than what modern readers would consider sexual slang today, but in Chaucer's time, these metaphors were lurid and graphic. The metaphors are now hard to understand, and from translation to translation, they differ in their severity. Some translations of The Canterbury Tales prefer to keep the old and hard-to-understand metaphors, while some modernize, and others restrict the metaphors even further as a measure of censorship. As a large part of the Canterbury Tales deals with sex in one form or another, most of the metaphors tend to shine through and are understandable. One can assume from context if an unknown metaphor is sexual or not.