(Notes: my information here is based largely on research I did in 1990; much of it comes from linguistics papers from the '60s and '70s. My information may therefore be somewhat outdated, but it's the most recent information I have available. On an unrelated note, you'll need a browser that can display tables to get the most out of this week's column. Apologies to those without such browsers; lining up stress marks without using tables would have been difficult.)
A couple of weeks back, I said that syllable stress was probably a combination of volume, pitch, and speed; on further checking my notes, however, I find that volume and pitch are indeed believed to be important, but evidence is not strong for duration being relevant.
Stresses can be perceived as a series of beats, forming a rhythm; rhythm is simply a pattern of stressed and unstressed syllables. (In poetry, such syllables are marked respectively with the symbols and .) Ordinary speech and prose contain rhythms, but most of the time those rhythms are irregular. When rhythms repeat in a more or less regular pattern, you have poetic meter.
Given a line of poetry with a regular rhythm, you can break up that rhythm into a series of repetitions of a short rhythm pattern; this repeated sub-pattern is called a metrical foot. Feet are categorized according to their rhythm; for instance, a two-syllable foot consisting of an unstressed syllable followed by a stressed syllable () is called an iamb. (Note that feet are determined by syllables, not by words; a foot can include part or all of one or more words.) You can describe the meter of a poem by indicating what kind of foot is used and how many feet are in each line; iambic pentameter, for instance, is poetry in which each line consists of five iambic feet. Here's a sample of iambic pentameter, the fourth line of Shakespeare's Sonnet 29:
You probably learned that much about meter in high school English class; you may not have learned that there are quite a few less common kinds of feet. Here's a list of all the named feet listed in Wood's rhyming dictionary (a name in italics indicates the foot is relatively common in English poetry):
Poetry should always be read with the rhythms that the same words would have in prose; never change the stress of a word to fit what you believe the rhythm to be. For instance, it would be inappropriate to read the first line of Sonnet 29 like this:
It would be more natural to read the line with stresses something like this:
You probably learned in English class that master poets don't always stick to a strict metrical pattern; they vary the meter of their poems, in order to draw attention to a line or phrase, or just to make the poem more interesting or less sing-songy. There is a certain set of allowable deviations from the meter; for instance, in iambic pentameter it's considered acceptable to replace the first iamb on a line with a trochee (as above), or to leave out the first or last syllable of the line. (Verse with such missing syllables is called catalectic; verse with a full syllable complement on each line is called acatalectic. Don't worry, there won't be a quiz on this terminology.)
Linguists, however, look at the set of allowable deviations and think "There must be some underlying linguistic pattern here, determining which deviations are allowable and which aren't." Given a new deviation, how else (other than by saying "nope, it's not on the list") could a poet determine if it's allowable? The linguistic idea of poetic metrics is that a line written by a poet as iambic pentameter and considered by knowledgeable readers to be iambic pentameter is iambic pentameter, regardless of its stress pattern. In particular, lines of iambic pentameter written by the great poets who used that meter, such as Chaucer, Milton, and Shakespeare, should be considered metrical ("correct") iambic pentameter.
A traditionalist might say that Shakespeare's line
(from King Lear) and Chaucer's
are extremely deviant lines, in which every iamb has been replaced by a trochee (or a pyrrhic). These could be considered catalectic lines, with a missing initial syllable, but then there's an extra syllable at the end to be accounted for...
A seminal paper in metrical phonology by Halle and Keyser (1966) provided a set of linguistic rules for determining whether a line of iambic pentameter in English is metrical or not. Subsequent papers (by various authors) refined and greatly extended this theory, but the gist remained the same. In Halle and Keyser's theory, a line of iambic pentameter consists of ten "positions" for syllables (usually, though a little confusingly, labeled alternatingly "W" and "S"; these labels originally stood for "weak" and "strong," but the labels do not mean that a W position must be filled with a weakly-stressed syllable, nor that an S position must be filled with a strongly-stressed one) followed by two optional "extrametrical" positions (usually labeled "(X)"). An entire line thus looks like this:
W S W S W S W S W S (X) (X)
Each of these positions can be filled by one or more syllables. Some (notably the first one and the two extrametrical ones) can be filled with zero or more syllables. More detailed rules determine what sorts of syllables, and how many, can be assigned to each position.
So by Halle and Keyser's rules, even a line like "Never, never, never, never, never" is completely metrical; it's simply a headless line (the first position is filled with zero syllables) with one extrametrical syllable at the end.
Reference: Morris Halle and Samuel J. Keyser (1966), "Chaucer and the Study of Prosody," College English XXVIII, 187-219; reprinted in Linguistics and Literary Style (Holt, Rinehart and Winston, 1970.)
Further references available on request.