If you paid any attention at all in high school English, you probably remember iambic pentameter, most likely from reading Shakespeare, and perhaps even other meters like trochaic tetrameter (the meter of the Teenage Mutant Ninja Turtles theme song, among other things). And if you had an English teacher who was especially instructive in etymology, you may have learned that iambic pentameter takes its name from several Greek roots that translate roughly as "five metrical feet." But wait. Greek and English meter don’t work in the same way, so how did we come to use Greek poetic terminology to describe English verse?
First off, we come by Ancient Greek metrical tradition, like marble statues and democracy, by way of the Romans. Fortunately, Greek and Latin share a few linguistic traits that made it relatively easy to borrow from one to the other. Both languages, for example, have a contrast between long and short vowels: es means "you are" in Latin while ēs means "you eat" (the ē is held for about twice as long as the e). For poetry, syllables with a short vowel and no consonant after it are "light," while syllables with a long vowel and/or a subsequent consonant are "heavy." Greek and Latin poetry, then, are based on the organization of light and heavy syllables into feet. For example, light-heavy is an iamb, heavy-light is a trochee, heavy-light-light is a dactyl, and so on.
The terms make sense in Greek: iamb comes from iaptein "to assail" (in words), literally "to put forth," since it was the meter of comic verses, while trochee comes from trokhaios (pous), literally "a running (foot)," from trekhein "to run." Dactyl is related to the word for finger, because heavy-light-light is like three joints, while anapest is literally "strike backwards," because light-light-heavy is the reverse of a dactyl. And of course the numbers—trimeter, tetrameter, pentameter, hexameter, etc.—are all from the Greek numerals.
So what does meter based on heavy and light syllables look like? Let’s take Latin as an example, since the Greek alphabet is harder to follow. Here’s an example from Catullus' Carmina 25, with light syllables marked by u, heavy syllables by –, and one foot per cell. So that's seven iambic feet per line (count 'em!) with a bonus heavy syllable at the end. And check out those lōng vōwēls!
|u –||u –||u –||u –||u –||u –||u –||–|
|Cinae-||de Thal-||le, mol-||lior||cunī-||culī||capil-||lō|
|u –||u –||u –||u –||u –||u –||u –||–|
|vel an-||seris||medul-||lulā||vel ī-||mul(ā) ōr-||icil||lā|
But English doesn't have long and short vowels in the same way. (You may have learned in school that the "i" in "bit" is short and the "i" in "bite" is long, but that’s not about duration; in other words, "bite" doesn't become "bit" if you just say it really quickly.) The lack of long and short vowels means that English can't have heavy and light syllables by the same definitions as Latin and Ancient Greek. So when English poets started looking to the classics for metrical inspiration, they were stumped. Without the building blocks of heavy and light syllables, they couldn't construct even the most basic of feet.
What English did have instead of long and short vowels was stress. Not the kind of stress that poets felt when they despaired at successfully imitating Cicero, but the kind of stress that can result in you putting the em-PHA-sis on the wrong syl-LA-ble.
In fact, English poetry was historically based entirely on stress, and we can still see this in certain nursery rhymes. For example, here's Hey Diddle Diddle (a poem that, incidently, can be translated into ASL) with the stressed parts bolded and with syllable and stress counts per line.
|Hey diddle diddle, the cat and the fiddle||11 syllables||4 stresses|
|The cow jumped over the moon||7 syllables||3 stresses|
|The little dog laughed to see such sport||9 syllables||4 stresses|
|And the dish ran away with the spoon||10 syllables||3 stresses|
If we try to assign a meter to Hey Diddle Diddle based on syllables, we get 11-7-9-10, which really doesn't look like a poem at all, and there's no real consistency with respect to how many unstressed syllables are between each of the stressed ones. (For reference, any line of iambic pentameter would have 10 syllables, because it contains five iambs of two syllables each.) But if we count just the stressed bits of each line we get the pleasingly regular pattern 4-3-4-3.
Also, stressing a syllable makes it sound not just louder, but longer. Remind you of anything? It definitely reminded the classics-crazy English poets of something, because they decided that stress in English was a reasonable substitute for heavy—long-vowelled—syllables in Latin and Ancient Greek. Sure, the Ancient Greek metrical tradition isn't quite as universal as the relative lengths of triangles. But we've managed to adapt it to English just enough that schoolchildren will be forgetting what exactly iambic pentameter is for generations to come.