Poe's concept of a "single effect" applies to short stories, and basically states that every element of a story should contribute to a single emotional effect of the story. This rule was important, because Poe was the founder of the short story, and this consequently became a foundational rule of the genre.
Poe had a number of different criteria that contributed to the single effect of the short story, explains PoeMuseum.org. He believed short stories should be able to be read in one sitting. This often contributes to the story's effect by allowing the reader to experience the complete effect of the story all at once, rather than leaving the story and having to regain the feeling of being in the story after coming back. His stories strove for a unifying emotion that draws the story together. Although Poe's effect was nearly always horror, other effects are possible. Poe also believed the effect of the story should be obvious and felt by the reader, even in the first sentence of the story. Finally, Poe aimed for truth in human behavior. He often wrote of bizarre and impossible-seeming events with little to do with reality, but he believed the human reactions to these events should mirror how people would likely react.