The line is a fundamental unit in verse, carrying meaning both horizontally across the page and vertically from one line to the next.
From A Poet’s Glossary
The following definition of the term line is reprinted from A Poet's Glossary by Edward Hirsch.
A unit of meaning, a measure of attention. The line is a way of framing poetry. All verse is measured by lines. On its own, the poetic line immediately announces its difference from everyday speech and prose. It creates its own visual and verbal impact; it declares its self-sufficiency. Paul Claudel called the fundamental line “an idea isolated by blank space.” I would call it “words isolated by blank space,” because the words can go beyond the idea, they can plunge deeper than thought. Adam Zagajewski says, “Tragedy and joy collide in every line.”
“Poetry is the sound of language organized in lines,” James Longenbach asserts in The Art of the Poetic Line (2008). “More than meter, more than rhyme, more than images or alliteration or figurative language, line is what distinguishes our experience of poetry as poetry, rather than some other kind of writing.” There are one-line poems called monostiches, which are timed to deliver a single poignancy. An autonomous line in a poem makes sense on its own, even if it is a fragment or an incomplete sentence. It is end-stopped and completes a thought. An line carries the meaning over from one line to the next. Whether end-stopped or enjambed, however, the line in a poem moves horizontally, but the rhythm and sense also drive it vertically, and the meaning continues to accrue as the poem develops and unfolds.
In “Summa Lyrica” (1992), Allen Grossman proposes a theory of the three modular versions of the line in English:
1. Less than ten syllables more or less.
2. Ten syllables more or less.
3. More than ten syllables more or less.
The ten-syllable or blank verse line provides a kind of norm in English poetry. Wordsworth (1771–1850) and Frost (1874–1963) both perceived that the blank verse line could be used to give the sensation of actual speech, a person engaging others. “The topic of the line of ten is conflict,” Grossman says, which is why it has been so useful in drama, where other speakers are always nearby. It has a feeling of mutuality. In the line of less than ten syllables, then, there is a sense that something has been taken away or subtracted, attenuated or missing. There is a greater silence that surrounds it, a feeling of going under speech, which is why it has worked well for poems of loss. It has also proved useful for the stripped-down presentation of objects, what the Imagists called “direct treatment of the thing.” We feel the clutter has been cleared away to create a clean space. Poems with drastically reduced lines aspire to be lyrics of absolute concentration, rhythmic economy. The line of more than ten syllables consequently gives a feeling of going above or beyond the parameters of oral utterance, or over them, beyond speech itself. The long lines widen the space for reverie. “The speaker in the poem bleeds outward as in trance or sleep toward other states of himself,” Grossman says. This line, which has a dreamlike associativeness, also radiates an oracular feeling, which is why it has so often been the line of prophetic texts, visionary poetry.