A distinction which i often have found elided in a lot of prosody discussions when they happen at all: between "stressed" (which is used as a binary with "unstressed") & "ictus" (beat) which is the thing counted.
Thus speaking a line more slowly may mean that a relatively unstressed syllable can be felt enough to count as a beat; & speaking a line more quickly (as in duple meters) may mean that a shortish stressed sylllable (in best practice) can be skipped between stronger beats. Not only vowel length but even the sound one word ends on & the next begins with--a notion we don't even have a name for so i invented one: "dit'tink"--can affect stressing, which is not after all an absolute, fixed thing. The way meter is taught (by that binary analysis which is based on Classical prosody vocabularies originally quantitative in origin) seems foolish & leads to formal poetry being read (& hence written) in a singsong way that does injury to both vernacular usage & the artistic intentions of most traditional poets. Whereas if you look for where the ictus falls, you can find out the meaning of iambic pentameter, its music, the musics of the shorter meters which are often stanzaic; & finally the older music which underlies pentameter--for at least the first century or more*--that counts four beats with a pause between them: & this is the native English meter.
Just as the shadow of pentameter can be felt in free verse (& more rarely--as in the Cantos, of dactylic), so can adumbrations of four-beat counting or three-beat counting be traced in vers libre from its earliest appearance. Much vers libre composed in the last 70 years is simply chopped-up prose (as the poets themselves often reveal by not pausing for line endings when they read out loud)--which is fine by me BTW--: only the handful of poets who compose "for the ear"** in this period can be said to have used beats deliberately. If they spend much time reading old poems, then they will show reminiscences of it in their rhythms. I am happy to see anyone trying to work at this consciously. It may even lead to us being able to read traditional verse with enjoyment again.
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* how many pentameter lines can you find a caesura in? & of course they were influenced by French prosody, which required it long into the 19c
** & not for the eye: looking at how it lies on the page with a certain amount of white space to the right