Lyric, Iambic Tetrameter
"The Song of Wandering Aengus" is a lyric poem. It's told in the first person (through the perspective of Aengus). Lyrics, as our wonderful lit glossary tells us, are usually written in the first person, and they give us an insight into the speaker's thoughts and emotions. Here, Aengus' first-person perspective gives us a glimpse into his experiences and his love for the "glimmering girl."
The meter that the poem is written in is something called iambic tetrameter. Now, before you make for the exits, let us explain what this means, exactly. Anything that's designated as a "tetrameter" is a line of poetry that's made up of four feet ("tetra-" means four). A foot, then, is the base unit of a line's rhythm. In this, case, our unit is the iamb. An iamb is a two-syllable pair in which the first syllable is unstressed and the second stressed. It makes a daDUM sound (say "allow" out loud and you'll hear a real, live iamb).
So, iambic tetrameter just means that we've got four of these iambs hanging out in each line. Let's break down the first four lines of the second stanza as an example:
When I had laid it on the floor
I went to blow the fire a-flame,
But something rustled on the floor,
And someone called me by my name:
You should hear the same pattern throughout each line: daDUM daDUM daDUM daDUM. That's iambic tetrameter for you. What's more, this pattern holds pretty much for the entire poem. So this is a very tightly structured poem.
Something else that holds in the poem is each stanza's rhyme scheme. The same pattern of end rhyme remains consistent throughout, to create a scheme of ABCBDEFE. Let's look again at stanza 2:
When I had laid it on the floor A
I went to blow the fire a-flame, B
But something rustled on the floor, C
And someone called me by my name: B
It had become a glimmering girl D
With apple blossom in her hair E
Who called me by my name and ran F
And faded through the brightening air. E
Now, we know that the repetition of "floor" in lines 9 and 11 technically counts as a rhyme, but we're still going to say that this is in line with the other stanzas because those lines use the same word—and it's cheating if we repeat the same word (we think so, at least). Even if you did count that as a rhyme, the rest of this stanza matches up exactly with the rhymes going on stanzas 1 and 3.
So… why is this form so rigid? Well, we know from his body of work that Yeats was a form nut. He loved to write in strict, regular meter. More than that, though, this is a poetic re-imagining of a popular cultural myth, so Yeats probably wanted to emphasize that. He's offering his own poetic take on the myth of Aengus, and the strict iambic tetrameter on display here is a regular, constant reminder that we're encountering a poem.