I fear I am about to give you a lecture. Blame Alon, he asked for it (In the comments of this post. Sort of).
First things first. When we speak English, some syllables have stronger emphasis placed on them than others. By this I do not mean the same sort of emphasis that we would denote by italics. I mean the natural variations in stress that create the rhythm of our speech. Following Stephen Fry's convention in this lovely book, when I want to show you which syllables are stressed I shall write the stressed syllables in bold, like this:
The natural variations in stress that create the rhythm of our speech.
An iamb is a weak (unstressed) syllable followed by a strong (stressed) syllable. In ordinary speech I might be likely to pronounce it iamb -- but don't. As a way of remembering it, stress the second syllable: iamb. Iamb, iamb, iamb, iamb. That's the rhythm of iambic verse.
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss,
Though winning near the goal -- yet do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!
(Keats, Ode on a Grecian Urn)
It is worth noting that if you merely read a poem in your head, you are less likely to take note of which syllables are strong and which ones are weak. This is one reason (and not the only one) for getting into the habit of always reading poems aloud. Even just whispering the poem makes a real difference to what you can get out of it. So, please, when I quote lines of poetry in this post, read them aloud to yourself. And if you will do me the courtesy of reading my humble poems aloud when I post them, I shall bless you as a considerate and thoughtful reader.
The excerpt from Ode on a Grecian Urn that I gave above is written in iambic pentameter. That is to say, there are five iambic 'feet' in each line; five sets of 'weak-strong'. Iambic tetrameter, with four iambs per line, is also quite common:
They fuck you up, your mum and dad.
They may not mean to, but they do.
They fill you with the faults they had
And make some extra, just for you.
(Philip Larkin, This Be The Verse)
A trochee is sort of the opposite of an iamb: 'strong-weak' rather than 'weak-strong'. Trochaic verse is less common than iambic verse, but there are certainly examples out there. Longfellow's Hiawatha, for example, is written in trochaic tetrameter (the name of the title character may have had something to do with that).
And the smoke rose slowly, slowly,
Through the tranquil air of morning,
First a single line of darkness,
Then a denser, bluer vapor,
Then a snow-white cloud unfolding,
Like the tree-tops of the forest,
Ever rising, rising, rising,
Till it touched the top of heaven . . . (link)
Longfellow wrote Hiawatha in huge sentences; they kind of drag on, sometimes, in my opinion. However, if I was writing trochaic verse I think I would have a hard time not doing that, myself. Trochees are not very, well, conclusive; they end weakly. Iambic verse, by contrast, ends on a strong syllable and, as a result, tends to sound stronger as a whole.
Within iambic verse, a trochaic substitution occurs when you substitute a trochee for one of your iambic feet. It can occur at the beginning of a line, like this:
Here with a Loaf of Bread beneath the Bough . . .
(Edward Fitzgerald, The Rubaiyat of Omar Khayyam)
It can also occur in the middle of a line, at the beginning of a new phrase, like this:
To be or not to be: that is the question.
(Don't get confused by the extra syllable at the end, there; it's called a feminine ending, and it's another perfectly common variation)
A trochaic substitution in the middle of a line forces us to have two stressed syllables next to each other. This naturally creates a tendency to pause between the two strong syllables. As a result, it's generally bad form to use a trochaic substitution in the middle of a line in a situation where the pause would not be natural. This rule about where trochaic substitutions are allowed to go is nearly universal. However, the effects that can be produced by trochaic substitutions within those limits are startlingly varied.
A trochaic substitution can be a way to start things with a bang:
Out flew the web and floated wide;
The mirror crack'd from side to side;
"The curse is come upon me," cried
The Lady of Shalott.
(Tennyson, The Lady of Shalott)
Trochaic substitutions can fit nicely into a strong, purposeful rhythm, as in the conclusion of this poem by Wilfred Owen where he explains why he isn't writing jolly little poems about the ways in which war isn't so bad after all:
You shall not hear their mirth:
You shall not come to think them well content
By any jest of mine. These men are worth
Your tears. You are not worth their merriment.
(Wilfred Owen, Apologia Pro Poemate Meo)
Trochaic substitutions can be used in dramatic verse to break the rhythm of a character's speech to show agitation. There are several good examples in Macbeth, Act I, Scene VII. Here's Lady Macbeth, berating her husband with trochaic substitutions all over the place:
Was the hope drunk
Wherein you dress'd yourself? hath it slept since?
And wakes it now, to look so green and pale
At what it did so freely? From this time
Such I account thy love.
My favourite trochaic substitution, though, is nearly invisible. It's a lilt, a beauty mark, at the beginning of the fourth line of the following poem. Since the metre flows naturally on its own, I won't spoil the elegance of it by crudely marking it out.
She walks in beauty, like the night
Of cloudless climes and starry skies;
And all that's best of dark and bright
Meets in her aspect and her eyes:
(Byron, She Walks In Beauty)
Would you rather be a good enough poet to write that, or would you rather be pretty enough to have that written about you? I can't decide. Not that I would be foolish enough to have anything to do with mister 'mad, bad and dangerous to know' if he was still alive, but he did write awfully sexy poetry.
If Emily Dickinson is writing it, a trochaic substitution can even be soft and lingering:
Hope is the thing with feathers
that perches in the soul . . . (link)
It's startling, but entirely typical of poetry, that the rules can be so strict, and the results so varied. I find that knowing something of the ways in which metre can vary makes a big difference to the way I read poetry. In Shakespeare, the metre can sometimes amount to a stage direction to say a line in a particular way. In Wilfred Owen's most famous poem, Dulce et Decorum Est, I read it again after I'd learned to read metre and couldn't believe I had never noticed before the way the metre sputters and dies in the final line. When Ebonmuse posted some excerpts from Wallace Stevens' Sunday Morning, I knew full well I wouldn't have read them in the same even, powerful way if I wasn't sorting each pentametric line into iambic beats (with the occasional trochaic one).
It's worth knowing.