The single effect
Poe's principle of the single effect remains the short story's most useful structural concept: every element — character, setting, imagery, dialogue, pacing — should work toward one unified impression. This is not a rule about simplicity but about coherence: the short story that is working at full compression will have every element contributing to the same emotional and thematic destination. The craft of the short story is largely the craft of identifying what the single effect of this particular story is and then eliminating everything that does not serve it. The short story writer who knows their story's single effect can make decisions about every element: this detail stays because it serves the effect; that detail goes because, however interesting in isolation, it dilutes the focus.