They say things come in threes, but in this poem, they come in fours. There are four lines per stanza, and, with some variation, four beats, or stressed syllables, per line.
Though there's no regular rhyme scheme in this poem, it has a very round sound, because of all the repetition of sounds within it, including the sound "round" itself. We also hear a lot of the "ill" sound in "hill," and the sound of "air" repeating throughout the poem. (We'll be going into more detail about the sounds of this poem in the "Sound Check" section, but the most important thing to get down now is that there's no set rhyme scheme.)
The poem, however, tries to trick us into thinking that it has a particular meter, or rhythm scheme. It masquerades as iambic tetrameter, but if we pay attention, we find that's not always the case.
Iambic tet-who-meter? Okay, let's break it down. First we start with the iamb, which is a unit of poetry that basically means an unstressed syllable followed by a stressed one. So, you hear a sound like da DUM. (If you say "allow" out loud, you'll hear an iamb.) The tetrameter part means that each line is ideally made up of four sets of iambs (tetra is Greek for four). Got it? Great. Let's look at an example, with stressed syllables bolded and italicized:
And round it was upon a hill. (2)
There are four definite stressed beats in this line. But look closely and you'll see that in lines like the third, the rhythm doesn't hold up:
It made the slovenly wilderness (3)
"Slovenly" and "wilderness" both end with two unstressed syllables, which breaks the iambic tetrameter. Then there are lines such as line 4 and line 10, which completely break away from the pattern:
Surround that hill. (4)
So why would this poem so blatantly break its established rhythm? Well, this irregularity reminds us much more of the wilderness than of a well-wrought, predictable jar. In that way, it's like we're getting the fight of the wilderness against the regular pattern of the regularly shaped, conformed jar. The rhythm of the poem—and, importantly, the breaks with that rhythm—neatly reflect the deeper conflict that Stevens' anecdote is all about.