for poetry. The word pentameter (penta meaning five) denotes a five-beat or ten-syllable line. An iamb is a beat in which the stress falls on the second half of the unit, as in "taTUM, taTUM." Thus, the lines in the ode are all five beats with an emphasis on the second syllable in the beat.

We can hear this if we look at the first line in the poem:

O wild / West Wind, / thou breath / of Au / tumn's being

Note that it is split in to five units of two syllables each (we do have to cheat a little and slur "being" into one syllable).

The stress falls on the second beat:

O wild / West Wind, / thou breath / of Au / tumn's being

A more perfect example of the iambic pentameter occurs in the second line:

Thou, from / whose un / seen pre / sence the / leaves dead

We don't have to slur any syllables in that line to achieve the pentameter effect. We can also understand, using the iambs, why Shelley makes certain choices, such as to switch the normal order of "dead leaves" to "leaves dead": he wants to place the emphasis in that unit on the word dead, not leaves.

Shelley has achieved a remarkable feat in not only sustaining the pleasing regularity of iambic pentameter across all five sections of the poem but in sustaining the four tercets (three-line stanzas) and the rhyming couplet in each section.