Stress: The rhythm of poetry depends on rhythmic elements present in ordinary speech. In English, speakers give more emphasis or stress to some syllables than others. We learned what syllables to stress and what syllables to leave unstressed more or less unconsciously when we learned to speak. Meter is, in English, the organization of stressed and unstressed syllables.
How to Scan a Metrical Poem: Almost all English poetry from the Renaissance to the end of the nineteenth century is written in a meter.
There are four possible accentual-syllabic meters in English, based on the possible patternings of stressed and unstressed syllables. Two of them are based on rising rhythms, patterns that come to a stress at the end, and two on falling rhythms, patterns that begin with a stress. The names for them come from Greek. The Greek prosodists called each unit of the pattern a foot, as if the meter walked. Here are the four possible foot types in English:
Rising rhythms:
the iamb, or iambic foot 
the anapest, or anapestic foot 
Falling rhythms:
the trochee, or trochaic foot 
the dactyl, or dactylic foot 
The Greeks distinguished many other meters based on other patterns of syllables, but as a practical matter, the ear seems to hear all unstressed syllables as organized around one stressed syllable preceding or following it—or at least that’s the way writers in English have always thought about it. So there may be these four possible meters. But, in fact, almost all metrical poetry in English is written in an iambic meter or in a meter that has an iambic base. In practice, dactylic meters hardly exist because the unstressed syllables are subsumed by the rising rhythm of the next stressed syllable, and the anapest can be thought of as a skipping iamb.
In a line of Shakespeare’s iambic pentameter:
When to/ the ses-/-sions of/ sweet si-/ lent thought
certain syllables, when, ses-, sweet, si, and thought, receive an accent, but one foot—/sions of/—doesn’t contain a lexical stress. Meter supplies it.
How do you know which syllable to stress in /When to/ or /-sions of/? Normally the pattern imposed by the meter tells you—it tells you that /-sions of/ is a metrical foot in an iambic line and so you apply the pattern. Sometimes rhetorical stress tells you. /When to/ is a metrical foot in a line of iambic verse, so you know to consider the two syllables in relation to one another. But the lexical rules tells you that in speech “when,” an adverb, receives stress and “to,” a monosyllabic preposition, doesn’t. What do you do? The rules of speech rhythm trump the metrical rule. The pattern is iambic, but the foot is trochaic. This way of establishing a pattern and then upending it from time to time is called substitution. It’s very common in metrical poetry, but more common in some parts of the line than others.
RULES OF SCANSION
1. The first rule of scansion: If you think you hear a meter in a poem, it is probably an iambic meter; the first thing to do is to test this hypothesis by dividing the line into poetic feet.
Note: Ignore punctuation and meaning when you divide the line into feet. All you’re trying to do is determine whether or not a pattern applies.
Note 2: Don’t arbitrarily chop it into twos. There may be an extra syllable in the form of an anapestic substitution.
You are reading Robert Frost’s “Stopping by Woods on a Snowy Evening” and it sounds metrical to you. So you try to discover the pattern by dividing the first two lines into feet:
Whose woods these are I think I know.
His house is in the village, though;
and then you mark the stressed and unstressed syllables to see if Frost was using an iambic meter.
2. The second rule of scansion is that, once you have determined the division into feet, you only compare the level of stress in a syllable with the level of stress of other syllables in the same foot. For example, in the second line, once you have separated the phrase “His house is in” into two feet, /his house/ is in/, you only compare “his” with “house” and “is” with “in.” You don’t compare “in” with “house.” This is to make sure that you are marking the meter and not the rhythm.
Rhythmically, you may hear a line with three strong accentual stresses, followed by another line with three strong accentual stresses, but the stresses are not the meter. The relation of stressed and unstressed syllables is the meter. So even if the rhythm sounds like this to you:
Whose wóods these are I thínk I knów.
His hoúse is in the v́illage, thoúgh;
—the meter looks like this:
/Wh̆ose woóds/ these aŕe/ Ĭ thínk/ Ĭ knów./
/Hi̇̆s hoúse/ is̆ iń/ th̆e víl/ lağe, thoúgh;/
Often you will hear what feel like strong irregular emphases in metrical poems. The irregularities are strong, because once a pattern is established, you notice departures from the pattern. So, of course, poets learned that one of the best ways to get emphasis is to establish a meter and then vary it. You vary it by changing the iambic pattern to some other pattern. There are three kinds of substitution in iambic verse:
an anapestic foot 
a trochaic foot 
a spondaic foot 
Some people would add a fourth kind of substitution:
a double iamb 
This is to accommodate a very common conformation in English, the phrase based on preposition-article-adjective-noun, like “in a dark time” or “on a high peak.” This isn’t entirely necessary. You may think of
in a dark time
as a trochee—in a—and a spondee—dark time, but some poets have felt that the piling up of two strong stresses in the second pair of syllables mutes the effect of any comparative difference in the first pair of syllables and have felt, therefore, that calling this configuration a double iamb reflects something about iambic rhythm in English more accurately.
It is also not uncommon for a line of iambic verse to have a final unaccented syllable. If all the lines have a final unaccented syllable, you are in the presence of a rare poem written in a trochaic meter.
Prohibitions: Contrary to what you might read in some manuals of prosody, there are kinds of substitutions that don’t occur in iambic verse. There are no pyrrhic feet—a pair of unaccented syllables—because the pattern will impose relatively more stress on one syllable in any pair in an iambic line. There are no dactylic feet. A three-syllable pattern of stressed-unstressed-unstressed doesn’t exist in English because the rising rhythm will always assimilate the third weak syllable to the next stressed syllable rather than to the previous one.
And, though there are single accented syllables sometimes in the first position in a line or an unaccented single syllable at the end, there are no single syllable feet in the middle of a line. If you find a pattern that seems to demand one, rethink your perception of the pattern. (Yeats’s “Lake Isle of Innisfree” is the notable exception.)
The most typical substitution in an iambic meter is the use of a trochaic or spondaic foot at the beginning of a line for emphasis. The first line in Yeats’s “Sailing to Byzantium,” for example, has a trochee in the first position:
That is\ no coun\ try for\ old men.\ The young
You can tell it’s a trochee because the rhetorical emphasis of the demonstrative pronoun that requires it. There is a spondee in the first position in another poem by Yeats written in memory of two dead friends:
Dear sha\ dows, now\ you know\ it all,
The substitution called a double iamb often occurs at the beginning of a line because of the usual structure of prepositional phrases in English—preposition, article, adjective, noun: “in a dark time, on a high peak.” If you apply our second rule to these phrases, you would probably scan them this way—
\ iń ă\ dárk tiḿe\ \ oń ă\ hiǵh péak\
—because the prepositions get slightly more semantic emphasis than the articles. To an ear tuned to the phrase-rhythm and not to the meter, they sound, of course, like this—
iń ă dárk tiḿe on̆ ă hiǵh peák
—and some poets tend to think of that pattern as a form of two-foot metrical substitution, in effect, a double iamb, so that you could, for example, scan a line from Robert Frost’s “Birches”—
He always kept his poise
to the top branches, climbing carefully
—like this:
\tŏ th̆e tóp brán\ ch̆es, cliḿb\ in̆g cáre\ fŭllý
3. So, the third rule of scansion is to be alert to substitution. For example, look at these lines from Frost’s “The Oven Bird”:
There is a singer everyone has heard,
Loud, a mid-summer and a mid-wood bird,
Who makes the solid tree trunks sound again.
He says that leaves are old and that for flowers
Mid-summer is to spring as one to ten.
First, divide it into poetic feet; then mark the stressed and unstressed syllables. You will have done well if you divided the lines like this:
/There is/ a sing/ er ev/ eryone/ has heard,/
/Loud, a/ mid-sum/ mer and/ a mid-/ wood bird,/
/Who makes/ the sol/ id tree/ trunks sound/ again./
/He says/ that leaves/ are old/ and that/ for flowers/
/Mid-sum/ mer is/ to spring/ as one/ to ten./
The only word you may have puzzled over is flowers. It is a word, like heaven, that is sometimes treated as a one-syllable word and sometimes as a two-syllable word, depending on how you hear and say it. Frost is either hearing it as a one-syllable word or he is leaving an extra unstressed syllable at the end of the line, as poets sometimes do.
But when it comes to scansion there may be several feet that give you pause. The first obvious substitution occurs in the foot /Loud, a/, in which it’s clear for both semantic and rhetorical reasons that “Loud” is more strongly accented than “a.” The pattern is stressed-unstressed, so the foot is a trochee, and this is a trochaic substitution. It’s the most common one in English verse.
You may also be unsure about how to mark the second and fifth feet in the same line. Is /mid-sum/ a spondee because both syllables are emphasized equally, or an iamb because sum gets slightly more emphasis than mid-? And what about /wood bird,/? This brings us to the fourth and fifth rules of scansion . . .
4. The fourth rule is that a tie goes to the meter. That is, the point of creating a metrical pattern is to draw the relation of stressed and unstressed syllables into an order. Ezra Pound had a metaphor for this order-making in art—“the rose in the steel dust.” He was thinking of the rose shape made when a magnet pulls random particles of steel dust into the pattern made by lines of electromagnetic force. Meter pulls syllables into its pattern in the same way. So, if you are not sure which way a scansion should go, give the meter the benefit of the doubt. In that second line above, therefore, it’s probably best to call both feet iambs. But if you think that the whole structure of sound is changed by those strong accents, then call them spondees.
5. Which leads us to the fifth rule: The point is not to be right; the point is to listen and to train your ear into the deepest textures of the sound of a poem. Once you notice what Frost is doing in that line, piling up strongly accented syllables—
Loud, a mid-summer and a mid-wood bird.
—it doesn’t too much matter whether you scan it
/Lóud, ă/ míd-súm/ měr ańd/ ă míd-/ wŏod biŕd./
or
/Lóud, ă/ míd-suḿ/ měr ańd/ ă míd-/ wŏod biŕd./
What matters is that you notice that Frost is trying to give you something of the energy of the bird’s song.
To summarize:
1. you determine the meter by inspection—by testing your sense of what it is until you discover what pattern the poet had in mind;
2. you mark stressed and unstressed syllables only by comparing the relative stress with other syllables in the same foot;
3. you are alert to substitution;
4. you assume, if you’re not sure, that the metrical pattern predominates;
5. and you don’t worry too much about whether you’re right or not; you listen to the sound of the poem.
METER LENGTHS
Poems with two metrical feet are written in dimeter. If in iambic feet, iambic dimeter. And so on. Three feet is trimeter, four is tetrameter, five is pentameter, six hexameter.
The commonest meters in English are iambic tetrameter and iambic pentameter.
PAUSES IN A LINE
When talking about the meter and the rhythm of a poem, it is often useful to pay attention to the changing position of the main pause in each line. The term for this pause is caesura, and caesuras are often marked with a double line, //. For example:
/There is/ a sing/ er//ev/ eryone/ has heard/
/Loud,//a/ mid-sum/ mer and/ a mid-/ wood bird./
You can see, if you imagine that brief silence moving around in relation to the unchanging measure of the ten-syllable line, how it would have a strong, subtle effect on the overall rhythm. And you should notice also that the caesura can occur either in a foot or at the end of one.
SCANSION: AN EXERCISE
Here are four whole poems and one excerpted passage from a poem by Robert Frost to practice scansion on. Each is written in an accentual-syllabic meter. I think they are in ascending order of difficulty for most people. Divide each into feet and mark the metrical pattern and the substitutions. In the fourth passage, mark the caesuras as well.
1. Stopping by Woods on a Snowy Evening
Whose woods these are I think I know.
His house is in the village, though;
He will not see me stopping here
To watch his woods fill up with snow.
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound’s the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
2. The Oven Bird
There is a singer everyone has heard,
Loud, a mid-summer and a mid-wood bird,
Who makes the solid tree trunks sound again.
He says that leaves are old and that for flowers
Mid-summer is to spring as one to ten.
He says the early petal-fall is past
When pear and cherry bloom went down in showers
On sunny days a moment overcast;
And comes that other fall we name the fall.
He says the highway dust is over all.
The bird would cease and be as other birds
But that he knows in singing not to sing.
The question that he frames in all but words
Is what to make of a diminished thing.
3. Neither Out Far Nor In Deep
The people along the sand
All turn and look one way.
They turn their back on the land.
They look at the sea all day.
As long as it takes to pass
A ship keeps raising its hull:
The wetter ground like glass
Reflects a standing gull.
The land may vary more;
But whatever the truth may be—
The water comes ashore,
And the people look at the sea.
They cannot look out far.
They cannot look in deep.
But when was that ever a bar
To any watch they keep?
4. Birches
When I see birches bend to left and right
Across the line of straighter darker trees,
I like to think some boy’s been swinging them.
But swinging doesn’t bend them down to stay
As ice storms do. Often you must have seen them
Loaded with ice a sunny winter morning
After a rain. They click upon themselves
As the breeze rises, and turn many-colored
As the stir cracks and crazes their enamel.
Soon the sun’s warmth makes them shed crystal shells
Shattering and avalanching on the snow crust—
Such heaps of broken glass to sweep away
You’d think the inner dome of heaven had fallen.
They are dragged to the withered bracken by the load,
And they seem not to break; though once they are bowed
So low for long, they never right themselves:
You may see their trunks arching in the woods
Years afterwards, trailing their leaves on the ground
Like girls on hands and knees that throw their hair
Before them over their heads to dry in the sun.
5. Design
I found a dimpled spider, fat and white,
On a white heal-all, holding up a moth
Like a white piece of rigid satin cloth—
Assorted characters of death and blight
Mixed ready to begin the morning right,
Like the ingredients of a witches’ broth—
A snow-drop spider, a flower like froth,
And dead wings carried like a paper kite.
What had that flower to do with being white,
The wayside blue and innocent heal-all?
What brought the kindred spider to that height,
Then steered the white moth thither in the night?
What but design of darkness to appall?—
If design govern in a thing so small.