What is the importance of the window in this short story?"The Open Window" by Saki

The expertise of Saki as a short story writer is certainly
apparent in his "The Open Window" as he employs the clever use of the window as the
symbolic perimeter of the frame story that Mrs. Sappleton's niece concocts in order to
exploit the nervous condition of Framton Nuttel.  This use of the window as the setting
of the narrative of the niece is particularly effective because an open window suggests
with its openness a certain truthfulness and candor.

Thus,
by employing the window from which Mr. Sappleton and his son have departed to hunt as
the frame within which she fabricates her story, the niece lends realistic detail and
veracity--notice her name, Vera--to her tall-tale so much so that she has effected
a blurring of the distinction between imagination and reality.  Because this condition
exists, when the two men do appear and pass through the open window, the nervous Framton
is horrified at what he perceives as ghosts. Clearly, the use of the open window is the
all-important device for Vera's "romance at short notice."