Since poets are now free to irregularly change the rhythms and sounds throughout a poem, they have many more choices to make with every word put on the page. T. S. Eliot said in his essay "The Music of Poetry" in 1942 that "no verse is free for the man who wants to do a good job", and, although written 60 years ago, it still holds true. The early 20th century changed forever the way we look at poetic form, but the traditions of sound and meter still hold a firm place in the poetic arts.
The words sound and meter are difficult to define and have many different aspects. Because of these difficulties, perhaps it is useful to think of these terms in the language of metaphor. If you think of the aural elements of a poem in terms of musical notation, you could think of meter as the rhythm created by the words (the horizontal movement of a piece of music, cutting up time into bigger or smaller increments) and sound as the notes of the piece of music (or the vertical movement, repeating sounds and syllables to create a "melody.") Each of these two elements are complex and require an in-depth definition. First, let's start with meter.
To "meter" something is to "measure" it (the word meter itself is derived from the Greek for measure), and there are four common ways to view meter.
Syllabic: A general counting of syllables per line.
Accentual: A counting of accents only per line. Syllables may vary between accents.
Accentual-syllabic: A counting of syllables and accents.
Quantitative: Measures the duration of words.
Of the ways of looking at meter, the most common in English are those that are accentual. English, being of Germanic origin, is a predominantly accentual language. This means that its natural rhythms are not found naturally from syllable to syllable, but rather from one accent to the next. There may be one, two, or three syllables between accents (or more, but this is a matter of debate). For this reason most English language poets opt to look at their own meter as accentual or accentual-syllabic. The former is the more common; adherence to the latter often leads an English language poet toward self-conscious verse, as their predictable rhythms are counter to natural English speech (not that it is impossible to create great verse with this technique, but there is a tendency for it to end up so).
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