While you were reading this poem, you should have noticed a certain bounciness in the language of the lines. That, Shmoopers, is no accident. That's meter at work! But how does it work? Let's take a closer look, shall we?
First off, the meter of "A Poison Tree" varies quite a bit. The most basic type of meter found here is something that folks in the know would call iambic tetrameter. What's that? Well, it's a type of meter in which there are four (tetra- is Greek for four) iambs. (An iamb is pair of syllables in which the first is unstressed and the second is stressed.) So, for example, check out line 2:
I told my wrath, my wrath did end.
When you read this out loud, you should hear four iambs together: da DUM da DUM da DUM da DUM. Still, lines 2, 4, 14, and 16 are the only pure iambic tetrameter lines in the poem. The other lines in the poem can be seen as deviations from this basic meter.
Take, for example, line 1:
I was angry with my friend.
The first syllable in this line is stressed, followed by an unstressed syllable. That's called a trochee, which is pretty much the opposite of an iamb. You will also notice that there are only seven syllables in this line, which means there can only be three syllable pairs (in this case, three trochees), with one leftover syllable. So, technically we would have trochaic trimeter, with one oddball, stressed syllable at the end. The process by which a metrical pattern is cut short like this (it can be at the beginning or the end) is called catalexis (just picture a cat driving a Lexus). Most of the lines in the poem have seven syllables and adhere to this pattern.
But why would Blake do this? Well, if you think about it, this poem is about the speaker and the speaker's enemy. It makes sense, then, to render the lines in two main metrical forms: iambic tetrameter and trochaic trimeter. We also noted that a trochee is the opposite of an iamb, so even the metrical bases of these lines seem at odds, the same way the speaker and the enemy are. And then we have the one extra beat at the end of the trochaic lines, which is a kind of bump in the rhythm of the poem, a disturbance of a sort.
Now, let's see. What else is added to the mix in the speaker and his enemy's conflict? Aha! The anger-apple. We can see how this extra bump at the end of the line might be a purposeful reminder of the key disturbing element in the poem: the speaker's unresolved anger.
The poem also rhymes, and therefore it has a rhyme scheme. In this poem, the rhyme scheme is: AABB. This means that in each four-line stanza, the first two lines rhyme (their rhyming sound is indicated with an A), and the last two lines rhyme (B). These rhyming pairs are called couplets, probably because they are like little verbal couples. And when you put two couplets together, you get a four-lined stanza called a quatrain.
Now, we know what you're all thinking: "This poem seems so simple at first!" The language really isn't difficult, the sentences are pretty basic, and in general the effect of the rhyme scheme makes it seem kind of like a children's poem. In reality, that's what it is partly meant to resemble (Blake once said that he thought his ideal readers were children, by the way).
But don't let the sing-songy rhymes fool you. Blake is notorious for wrapping up complex ideas in very easy-to-unwrap packages. This union of simplicity and complexity proves that even supposedly "simple" things can be vehicles for profound observations about human nature. In a way, this approach allows for multiple levels of appreciation. You can simply sit back and enjoy the great sounds of the poem and its rhyming, or you can dive deeper, and really plumb its depths for meaning. Can you guess which option we chose? Right! What, did our scuba suit give us away?