What precise function has the open window in "The Open Window"?

The open window leading from the big living room to the
outdoors provides unity of place, to use one of Aristotle's terms. In plotting the story
Saki wanted to have the visitor Framton Nuttel terrorized by the arrival of three men
carrying guns, three men whom he thought to be either ghosts or "living dead," zombies.
Saki decided that he could not have the three hunters return through a back door and get
rid of their guns and dead birds before entering the living room, because that would not
elicit the desired effect on Framton Nuttel. The open French window was essential to the
story. That was why Saki titled his story "The Open Window" and used so many words to
explain why it was open and what Vera and her aunt were expecting to see. When Vera
first greets Framton, she says:

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"You may wonder why we keep that window wide open
on an October afternoon."

It
is actually the reader who may wonder why they keep the window wide open, and Saki,
through Vera, has to explain as best he can. The reader may also wonder why these men in
their wet clothes and muddy boots don't customarily reenter the house by a back door and
take off their boots before coming into the living room for tea. Saki had to make it
plausible that the men should be so careless. The reader is led to believe that that's
just the way upper-class country gentry behaved. They even brought their muddy spaniel
straight into the living room, and the dog probably spent much time lounging on the
furniture. The three men and their dog are not only going to come straight into the
living room, but we the readers are led to believe, by both Vera and Mrs. Sappleton,
that they always come straight into the living room through the open French window,
regardless of how wet and muddy they might have become. Note what Mrs. Sappleton says to
Framton when she sees the men outside.

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"Here they are at last!" she cried. "Just in time
for tea, and don't they look as if they were muddy up to the
eyes."

Moments earlier she
makes a similar remark:

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"I hope you don't mind the open window," said
Mrs. Sappleton briskly; "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?"

Mrs. Sappleton has led a
sheltered life. She assumes that Framton must be interested in killing birds and capable
of tracking mud all over people's floors because she takes it for granted that men are
all alike. Three men and a dog will be tracking mud all over her carpets and hardwood
floors, but she doesn't seem perturbed. Saki will have to persuade the reader that the
three men don't care about making a mess and Mrs. Sappleton doesn't care either, because
Saki wants them to enter the living room directly from the outside. That is the only way
that Framton could see them heading straight towards him carrying their
guns.

Once Saki thought of having the men return through an
open French window, he used it to inspire Vera to make up her story. It was essential to
Vera that her aunt should be in the habit of waiting for the men every evening at tea
time and that she should be expecting them to enter through that window. Saki still
doesn't explain why the French window, which is like a floor-to-ceiling door, should
have to be standing open. The men ought to be capable of opening the window for
themselves. It can't be that warm inside the room if the men are returning all wet and
muddy and it is getting dark outside. A French window is made mostly of glass. It is
more of a window than a door, but it can be used as a door. The window really did not
need to be "open" at all for Framton to see the men approaching, but the fact that it is
open is needed to establish that this is the way the men will be entering the house. The
fact that it is open makes their approach more frightening to poor Framton because it
eliminates the faint possibility that the window might be
locked.

Vera's story is especially effective because these
three men are avid hunters. So all three are carrying guns. This makes them infinitely
more frightening to poor Framton.