Why is the speaker’s bed nailed down?
The speaker's bed is nailed down because she is not staying in the nursery of an old house, as she believes, but because she is staying in a room used to treat women who are suffering from hysteria. There are lots of strange details about the room itself. She says that there are "rings and things in the walls," there is a gate at the top of the steps that she cannot open and shut, and there are bars on the windows. If it were an nursery, furthermore, there would be no need for a big bed; there would be a crib instead.
Moreover, it seems as though the narrator is being treated with the rest cure, pioneered by Weir Mitchell -- the doctor who is mentioned in the story and who treated Gilman as well -- and this means that she is to get, as she says, "perfect rest." She is not to work, to write, to read, not even to think. At one point, she mentions a rope that Jennie, presumably her sister-in-law, forgets to take, and this makes it seem as though the narrator is actually being tied down in her bed: a common practice if women would not submit to "perfect rest" during their cure. It might be necessary to nail the bed down if the narrator is going to be confined to it and might thrash around. She also says that the wallpaper is "scratched off about as far as [she] can reach" while lying on the bed, so it makes sense that she may be confined to it at times.
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