What did Edgar Allan Poe mean by the unity of effect in a short story?

For Edgar Allan Poe, the "unity of effect" was the definitive characteristic of a successful short story. He argued that a skilled artist should first conceive of a specific, unique effect to be achieved and then carefully construct the narrative to serve that singular purpose.1

According to Poe, every element of the story—from the initial sentence to the final word—must contribute directly or indirectly to this pre-established design. If a writer fashions their thoughts merely to accommodate incidents rather than working toward a unified goal, they have failed in their artistic duty.1

This principle emphasizes craftsmanship and artistic integrity over specific subject matter or technique. By requiring that every word and event be essential to the intended effect, Poe sought to ensure that the composition possessed a oneness of tone, structure, and movement. This approach allowed his work to maintain a powerful, focused impact on the reader, whether he was exploring themes of terror, logic, or the macabre.12