While "I dwell in Possibility – " might look mild mannered on the page, it's anything but tame. At first glance, we see three tidy quatrains (a.k.a. stanzas with four lines). Nothing crazy there. Even the first couple lines seem steady rhythmically.
The first line is in iambic tetrameter. What's that? Well, an iamb is an unstressed syllable followed by a stressed one. Read it out loud, accenting the syllable in bold, and you'll see what we're getting at here:
I dwell in Possibility (1)
We say this line is in tetrameter because it has four iambs in a row (tetra = 4). Simple, right? Let's see how the next line compares...
A fairer House than Prose – (2)
Yeah, pretty similar, but this time we've only got three iambs, meaning that this line is in iambic trimeter (tri =3).
If the poem alternated between tetrameter and trimeter the whole way through, it would be in what's called ballad meter. This is a style of meter popular in church hymns—especially ones written by Isaac Watts, whose hymns were big hits at the church Dickinson attended when she was growing up.
Possibilities of Meter
Here's the thing, though: the rest of the poem doesn't stick to this meter at all. From here on out, the poem really lets its hair down. Check it:
More numerous of Windows –
Superior – for Doors – (3-4)
In line 3 we don't go back to tetrameter; instead, we've got what looks like iambic trimeter with an extra unstressed syllable on the end. After that we go back to real deal trimeter for line. The first two lines of the second stanza do the same thing:
Of Chambers as the Cedars –
Impregnable of eye – (5-6)
We've got wonky almost-trimeter in line 5 and genuine trimeter in line 6. After this the next two lines swing back around to the hymn meter pattern we started with:
And for an everlasting Roof
The Gambrels of the Sky – (7-8)
See how the we've got iambic tetrameter in line 7 and trimeter in line 8? Okay, enough of this.
Why So Wonky?
So what is the point of starting a poem off in a steady meter and then getting all funky with it? Was Dickinson just being lazy? Did she just not care a whole lot about meter? Nobody knows for sure. But if you ask us—which we're assuming you're doing—this shift from order to barely contained chaos feeds into the larger meaning of the poem.
It's a piece about the limitless power of poetry to awaken human imagination, so why would the whole thing it be written in an super constricting meter? Instead, its starts there, then gets wild. Physically speaking, a poem is nothing but words printed on a page; it's finite like the first two lines of this particular poem. After a poem is read, though, it explodes into the dimension of the mind.
So in a way the rhythmic structure of this poem mimics the act of reading the poem, which is particularly crazy because it's a poem about the power of poetry.