The majority of The Book of the Duchess was written in rhyming couplets (AA, BB, CC, DD, and so on). But so many changes have occurred in pronunciation and stress between Chaucer's Middle English and today's English that we tend to read the lines with more or fewer (and different) syllables than Chaucer intended. The rhymes, too, are often obscured, primarily because of the arbitrary nature of the rules applying to silent letters in Modern English compared to Chaucer's English. Still, the rhymes in these eight-syllable lines are usually decipherable.
A common example of such differences regards verbs ending with -ed. "Naked" and "maked" both rhymed and had two syllables when Chaucer was writing. Likewise, -le at the end of words such as "wele" (well) and "whele" (while) was generally pronounced as a distinct syllable, which would contribute to the count of the line and the end rhyme. In general, as above, more letters were pronounced in Chaucer's English than are pronounced today.
In some cases the rules for consonants are more regular in Middle English. "Caughte" (in some editions "Kaughte") and "draughte" (lines 681 and 682) would have been pronounced far more similarly in Chaucer's time and would have been more than just a visual rhyme, in comparison to the modern pronunciations of "caught" ([cot]) and "draught" ([draft]).
Some knowledge of the pronunciation of Middle English is helpful in discerning the rhymes and enjoying the regularity of the meter, but Chaucer's poem can be well appreciated with simply the help of a good Middle English glossary (see Related Links). The poetic form of The Book of the Duchess is surprisingly consistent without feeling artificial or inflexible. Chaucer's rhymes are often jokes in themselves.
The eight-syllable line works well for the length of this poem and the subject matter. The couplet form only rarely veers into the ridiculous, usually intentionally on Chaucer's part, while the narrator is speaking, making that character look appropriately ridiculous. The language and the richness of the story and the references are showcased well in the form Chaucer chose.