setting. Vera talks about the open window. The aunt immediately begins talking about the open window. Framton Nuttel remains uncomfortably aware of the open window, and the three supposedly dead men are obviously intending to enter the house through that open window.Saki does not explain why the French window has to be left standing open. Couldn't the hunters just open the window themselves? Is it possible that the window is designed in such a way that it can only be opened from the inside because there is only an inside handle? That seems plausible. It would allow people on the inside to open it but prevent outsiders from doing so. It would provide both convenience and security. Readers of Saki's time might have understood this without explanation.

Saki's use of the open window is a marvelous technique for so many reasons, some of which have already been mentioned.  Clearly, it is an ironic symbol as an open window usually indicates truth and the future.  In Vera's case, it is the opportunity for fabrication and a closed tale of death.  It also suggests, as the above post mentions, a wariness of venturing out to areas unknown.

Let us not forget,also, the reiteration of the concept of the frame story.  Vera's tall tale is a framed story within the main story figuratively; it is also a literally framed story as there is the frame of the real window through which the imaginary tale grows.

I have always thought that the open window of the title acts symbolically to describe the opportunity that Vera sees when she meets Frampton and deduces that he is an "open window" for her talent for creating stories. In a sense, the story warns us about being to susceptible to the power of excellent story tellers, such as Vera. We are warned to not let ourselves become an "open window," exposed to manipulation, as Frampton himself was.

The title also represents the endless possibilities of Vera's imagination and Framton's fears. The window's being open allows for anyone or anything to enter into the seemingly safe house (remember that Framton has come to Vera's area of the country to calm his nerves).

Similarly, Vera--an observant, precocious teen--seizes upon the "open" opportunity to frighten Framton.

I would that it is because the open window is a major part of the story.  The fact that the window was open may well have given Vera the idea of making up that story about the hunters being dead.  The window made it possible for Framton to see the hunters as they returned.  Since the window makes the story possible, it makes sense to name the story after it.

The reason why Mrs. Sappleton keeps her window open is because her husband and her brothers go hunting quite a lot (shooting). That particular day, the shooting party had been out in the marshes and Mrs. Sappleton knew that they usually would come that route back to the house. Since she did not want them to stain the carpets, she would let the window open so that they could come through the window, rather than through the front door.

..."my husband and brothers will be home directly from shooting, and they always come in this way. They've been out for snipe in the marshes to-day, so they'll make a fine mess over my poor carpets. So like you men-folk, isn't it?

It is a practice which Mrs. Sappleton's niece, Vera, used to make up a story that would scare house-guest Mr. Frampton Nuttell away. It is precisely what it did for, when the men returned, they even asked who was it who bolted out of the house in a frenzy.

Here we are, my dear," said the bearer of the white mackintosh, coming in through the window; "fairly muddy, but most of it's dry. Who was that who bolted out as we came up?

The question of "why" an author writes a particular work is one of the most enticing yet one of the most difficult to answer. Usually, the only clues to "why" lie in the theme(s) of the story or in the traits of the characters. Sometimes, author biography and contemporaneous social issues might give further indications of "why" as with Dostoevsky and Dickens, for example. In some cases, genre might also give a clue about "why," such as when the literary piece is a satire, which is written to ridicule and reprimand a person, group or society for breaking society's accepted moral and ethical code.

In the case of "The Open Window," we actually have clues from genre, theme, characterization and author biography, which has been adequately speculated about by literary critics. First, the story is a satire of adults who are mentally unstable and/or gullible and foolish. So one reason he wrote this story is to show society a true picture of itself by ironically satirizing the harm being done, presenting mentally unstable Nuttel as both a cause and the victim.

Another clue is the themes, one of which is appearance versus reality. Saki shows how frightfully easy it is for "self-possessed" Vera to deceive and harm the stranger Nuttel and her familiar aunt and uncle, the Sappletons. Appearance versus reality also attaches to the introduction of the supernatural when the husband and younger brothers enter the yard and approach the open window sending Framton (a name indicating one who worries and is mentally unstable) running out the front door. Framton's flight raises a question about what happened to him afterward. This speculative question might be a clue to "why" Saki wrote the story. It certainly relates to what critics call the "enfant terrible":

According to Janet Overmyer, children in Saki's stories often are "cruel to adults because ... [they must] snatch their revenge whenever the opportunity arises." ("Saki's Enfant Terrible in 'The Open Window'." Gale Cengage on eNotes.com)

The enfant terrible introduces clues from Saki's biography. Saki often drew child characters like Vera who routinely and deliberately made havoc in the adult worlds they inhabited. Some critics suggest Saki was drawing a true picture of a type of perverse child and adolescent that he either was or had experience with. Some critics suggest Saki drew the perverse child mind because he never recovered from his oppressed childhood and was taking revenge against his caretakers.

To put all these clues together, it may be that Saki wrote this story because he wanted to improve the adult world he satirized and to warn against the perverse child mind he employed. Perhaps there is another possible way to put the clues together, but there seems to be a good case for this explanation.

"Their bodies were never recovered. That was the dreadful part of it." Here the child's voice lost its self-possessed note and became falteringly human. "Poor aunt always thinks that they will come back someday, they and the little brown spaniel ...."

Vera, Mrs. Sappleton's niece, has told the anxious Framton Nuttel quite a story about the open window. She explains that Mrs. Sappleton's husband and brothers had gone hunting three years ago and had drowned in the bog. Vera says that the men's bodies were never recovered. Framton's sister had known Mrs. Sappleton in the past, but it was more than three years ago since then, and so Framton has no reason not to believe Vera's story. Vera says that Mrs. Sappleton leaves the window open because she persists in believing that the men will return some night at dusk.

In reality, the window is left open because the husband and brothers of Mrs. Sappleton left to go hunting for snipe that very morning, and the men always come back into the house through it. We can assume, then, that they must be the type of windows that begin at or near the floor and extend upward quite high on the wall. When Mrs. Sappleton explains this to Framton, she seems to confirm Vera's off-putting tale (which seems "purely horrible" to the poor young man). When the men do return from hunting, Framton believes that they are ghosts and takes off running without a word to his hostess. We learn, finally, that "Romance at short notice" is Vera's specialty.