Why did Geoffrey Chaucer change from octosyllabic couplets to 10-syllable lines?

Geoffrey Chaucer’s transition from octosyllabic couplets to 10-syllable lines—specifically iambic pentameter—marked a significant evolution in his craft and a pivotal moment for English poetry.1

In his earliest substantial works, such as The Book of the Duchess and The House of Fame, Chaucer utilized the octosyllabic couplet, a meter of French origin that was fashionable for courtly love poetry at the time. However, he eventually abandoned this meter, viewing it as "jejune" or lacking in depth.1

Chaucer’s shift toward the 10-syllable line was likely influenced by French syllable-counting techniques. By developing a line of 10 syllables with alternating accents and regular end rhyme, he created a more sophisticated and flexible structure. This new form, which became the ancestor of the heroic couplet, allowed for greater complexity and range in his narrative verse, most notably displayed in The Canterbury Tales.21