If each line of a poem has a regular pattern or metre, we can call the line after its metre and its length. Thus a line of five feet, all iambics, would be referred to as iambic pentameter.
The standard length of line of the traditional English sonnet
The length of line of the heroic couplet, which are rhyming pairs of iambic pentameters, such as Pope's The Rape of the Lock
The length of line of blank verse, which is unrhymed iambic pentameters, used by Shakespeare and his contemporaries, in Milton's Paradise Lost, and in Wordsworth's The Prelude.
The predominence of iambic pentameter
There are two main reasons why the iambic is so common in English verse.
The requirements of rhyme
There is a preference in English poetry for masculine rhyme, that is, rhyme on a final stressed syllable. This means the final syllable will be stressed, and therefore the foot will have to be a spondee or a rising metre. Trochaic metres need feminine rhymes, where the last but one syllable is stressed and rhymes, but then the last unstressed syllable also has to rhyme, as in ‘falling/calling'. Where rhymes need to dominate, this is a less preferable option.
Iambic metre naturally takes the form of the normal English sentence:
English sentences typically start with an article (a, the, some) or possessive adjective (my, his), which would not be stressed
This is then followed by a noun or adjective; more often than not, the stress for these is on the first syllable if polysyllables.
Thus, a sentence would typically begin with an unstressed followed by a stressed syllable, the mark of the iambic.
‘The yellow puppy quickly followed at my heels'
– not an unusual word order or grammar.
If we scan this, it becomes:
‘The yéll|ow púp|py quíck|ly fóll|owed at my héels|.'
The iambic would demand a stress of some sort, even the smallest secondary, on ‘at', but it would be forcing the sound to put one there. On the other hand, many English poets like to give their iambic pentameters a bit of a rest like this, and not bother to really accent one of the middle feet, though the scansion needs it. It makes for variety and prevents too obvious a rhythm.
Older poets had a large selection of poetic diction to keep the metre regular, whereas Hopkins substituted diction of his own. He was moving away from strict regularity without quite moving into free verse, where all the old traditional patterns are simply discarded.
First foot inversion
Just having the first syllable of a line accented does not necessarily mean a falling metre. The iambic line frequently inverts its first foot, for emphasis or exclamation, for example. After the first foot, the metre returns to a rising one. Thus:
‘Crúshed, why| do mén| then nòw| not réck| his ród?' (God's Grandeur)
‘Lóok at| the stárs....' (The Starlight Night). Author G.M. Hopkins
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