Despite the extremely modest circumstances of its prepublication and of its first two editions, Banville’s Petit Traité de poésie française, his treatise on French versification, ended up, ten years later, at the heart of a quarrel concerning the direction French verse should take, between on the one hand the Parnassians, defending the purity of traditional verse as rejuvenated by Victor Hugo, and on the other the Symbolists and Decadents, who favoured free verse. Both sides defended its point of view heartily, relying on Banville’s authority. Their respective arguments reveal the impact the Petit traité had on the evolution of French verse in the 1880s.