Several different types of meter show up in this poem on the way to "Eldorado." In general, the first and second lines, and the fourth and fifth lines, of each stanza are written in a form called iambic dimeter. This means that each one of those lines contains two (the prefix di- means "two") iambs. An iamb is a beat or foot that contains an unstressed syllable, followed by a stressed syllable. (If you say the word "allow" to yourself, you'll hear an iamb. It sounds like this: da DUM.) Let's take line 2 as an example:
A gallant knight.
You should hear da DUM da DUM in that line. Sometimes, however, the lines are a little bit different. Take the line above line 2, line 1:
The first beat begins with a stressed syllable, instead of an unstressed syllable. This is the reverse of an iamb (it sounds like DUM da, instead of da DUM) and is called a trochee.
Now, the third and sixth lines of each stanza are a little more complicated. Let's examine one so we can see:
In sunshine and in shadow. (3)
You will notice that we have three iambs and one extra syllable, chilling out by its onesies there at the end ("-dow"). What's up with that? Well, we actually think this is kind of a genius move on Poe's part. In a poem that's about disappointment and fruitless question, this extra beat at the end of the line is a sonic reminder that things don't wind up nicely and neatly—for either the knight or us, the readers. There's always an extra disturbance to this regular iambic trimeter, in the way that Eldorado is always out of the reach for our poor knight. Neat, huh?
Now, his pattern holds for the first three stanzas of the poem, but things get a little tricky in the fourth stanza, where Poe pretty much starts playing games. Line 19 can't really be read as iambic dimeter or trimeter (even if read "o'er" as one syllable). Likewise, line 20 is missing a syllable that keeps it from being neatly iambic. And lines 21 and 24 don't feature the extra syllable we've been telling you about in stanzas 1 through 3. In fact, both of these lines contain exactly eight syllables and are packed with trochees, rather than iambs. (In fact, they are examples of trochaic tetrameter—a line with four trochees.)
Now, we know you want an answer to the burning, all-important question: Why in the wide world of sports does Poe change things so radically in the last stanza? Well, we don't have an answer, so good luck with that… J/K! We actually kind of do. Here's the deal: When everything is the same for quite a while and then it all of a sudden changes, it's pretty noticeable, right? Well, the same goes for poetry. The changes in the last stanza, while small, are definitely noticeable.
Poe does this, likely, so that we pay more attention at that point in the poem, so that we remember it. More than that, though, this is where the knight realizes that he'll never, ever find Eldorado. The neat rhythms that the poem has established now come off the rails. Even on a sonic level, then, the futility of the knight's quest, the fact that it will only end in death, becomes quite clear—to him and to us (and our ears).
You may also have noticed that Poe's poem rhymes all over the place, which means it has to have a rhyme scheme, a diagram that explains which lines rhyme with which. The pattern for the first three stanzas is this: AABCCB, where each letter represents the end rhyme of that last word in the line. This means that the first two lines rhyme, the third and sixth rhyme (that's the neat "Eldorado" and "shadow" rhyme), and the fourth and fifth rhyme. The two groups of lines that rhyme next to each other (those marked A and C) are called couplets. In the fourth stanza, the pattern is the same, except—interestingly enough—the first two lines (19 and 20) don't rhyme.
The choices Poe makes in setting up the poem's rhyme scheme go a long way toward reinforcing its central idea. For example, the structure of a rhymed couplet, followed by a line that ends in "shadow," then another couplet followed by a line that ends in "Eldorado," makes the poem kind of like a wave.
It goes towards shadow, then goes towards Eldorado. It swings back and forth, we might say. It's almost as if the knight is moving toward one, just as he is moving toward the other. It's like he's trapped: he's between the dream of Eldorado, and the grim reality of the shadow. Moreover, since these words rhyme, it subtly hints at the notion that they're equal in some way, as if the pursuit of Eldorado might be the exact same thing as pursuing an immaterial, dark nothingness—which, of course, the poem's ending only underscores.
That's the same idea behind the rhyming oddness found in the fourth stanza, which—just like the rhythmic oddness—calls our attention to the fact that our knight's quest has come undone by this point, just like the rhyme and rhythm. Poe's use of form and meter, then, are important, if subtle, reminders that this knight's tale has a super-sad ending.