A line is a unit of language into which a poem or play is divided, which operates on principles which are distinct from and not necessarily coincident with grammatical structures, such as the sentence or single clauses in sentences. Although the word for a single poetic line is verse, that term now tends to be used to signify poetic form more generally.
A distinct numbered group of lines in verse is normally called a stanza.
General conventions in Western poetry
Conventions that determine what might constitute line in poetry depend upon different constraints, aural characteristics or scripting conventions for any given language. On the whole, where relevant, a line is generally determined either by units of rhythm or repeating aural patterns in recitation that can also be marked by other features such as rhyme or alliteration, or by patterns of syllable-count.
In Western literary traditions, use of line is arguably the principal feature which distinguishes poetry from prose. Even in poems where formal metre or rhyme is weakly observed or absent, the convention of line continues on the whole to be observed, at least in written representations, although there are exceptions (see Degrees of license). In such writing, simple visual appearance on a page (or any other written layout) remains sufficient to determine poetic line, and this sometimes leads to the suggestion that the work in question is no longer a poem but "chopped up prose". A dropped line is a line broken into two parts, with the second indented to remain visually sequential.
Distinct forms of line, as defined in various verse traditions, are usually categorised according to different rhythmical, aural or visual patterns and metrical length appropriate to the language in question. (See Metre.)
One visual convention that is optionally used to convey a traditional use of line in printed settings (in languages represented by alphabetic scripts) is capitalisation of the first letter of the first word of each line regardless of other punctuation in the sentence, but it is not necessary to adhere to this. Other formally patterning elements, such as end-rhyme, may also strongly indicate how lines occur in verse.
In the speaking of verse, a line ending may be pronounced using a momentary pause, especially when its metrical composition is end-stopped, or it may be elided such that the utterance can flow seamlessly over the line break in what can be called run-on.
Degrees of license
In more "free" forms, and in free verse in particular, conventions for the use of line become, arguably, more arbitrary and more visually determined such that they may only be properly apparent in typographical representation and/or page layout.
One extreme deviation from a conventional rule for line can occur in concrete poetry where the primacy of the visual component may over-ride or subsume poetic line in the generally regarded sense, or sound poems in which the aural component stretches the concept of line beyond any purely semantic coherence.
At another extreme, the prose poem simply eschews poetic line altogether.
In every literature there is a metrical pattern that can be described as "basic" or even "national"[dubious ]. The most famous and widely used line of verse in English prosody is the iambic pentameter, while one of the most common of traditional lines in surviving classical Latin and Greek prosody was the hexameter. In modern Greek poetry hexameter was replaced by line of fifteen syllables. In French poetry alexandrine is the most typical pattern. In Italian literature endecasillabo that is a metre of eleven syllables is the most common line. In Serbian ten syllable lines were used in long epic poems. In Polish poetry two types of line were very popular, a 11-syllable one, based on Italian verse and 13-syllable one, based both on Latin verse and French alexandrine. Classical Sanskrit poetry, such as the Ramayana and Mahabharata, was most famously composed using the shloka.
- English iambic pentameter:
- Like to Ahasuerus, that shrewd prince,
- I will begin — as is, these seven years now,
- My daily wont — and read a History
- (Written by one whose deft right hand was dust
- To the last digit, ages ere my birth)
- Of all my predecessors, Popes of Rome:
- For though mine ancient early dropped the pen,
- Yet others picked it up and wrote it dry,
- Since of the making books there is no end.
- (Robert Browning, The Ring and the Book 10, Book The Pope, lines 1-9)
- Latin hexameter:
- Arma virumque canō, Trōiae quī prīmus ab orīs
- Ītaliam, fātō profugus, Lāvīniaque vēnit
- lītora, multum ille et terrīs iactātus et altō
- vī superum saevae memorem Iūnōnis ob īram;
- multa quoque et bellō passūs, dum conderet urbem,
- inferretque deōs Latiō, genus unde Latīnum,
- Albānīque patrēs, atque altae moenia Rōmae.
- (Virgil, Aeneid, Book I, lines 1-7)
- French alexandrine:
- Comme je descendais des Fleuves impassibles,
- Je ne me sentis plus guidé par les haleurs :
- Des Peaux-Rouges criards les avaient pris pour cibles,
- Les ayant cloués nus aux poteaux de couleurs.
- (Arthur Rimbaud, Le bateau ivre, lines 1-4)
- Italian endecasillabo:
- Per me si va ne la città dolente,
- per me si va ne l'etterno dolore,
- per me si va tra la perduta gente.
- Giustizia mosse il mio alto fattore;
- fecemi la divina podestate,
- la somma sapïenza e ’l primo amore.
- (Dante Alighieri, Divina commedia, Inferno, Canto III, lines 1-6)
- Line break (poetry)
- Active listening
- Part (music)
- Run-on sentence
- Principles of organization
- Repetition (music)
- Canons of page construction
- Graphic design
- See, for example, the account in Geoffrey N Leech A Linguistic Guide to English Poetry, Longman, 1969. Section 7.3 "Metre and the Line of Verse", pp.111-19 in the 1991 edition.
- See  for an example.
- Metre, prosody at Encyclopaedia Britannica
- Hexameter, poetry at Encyclopaedia Britannica.
- Alexandrine, prosody at Encyclopaedia Britannica
- Claudio Ciociola, Endecasillabo at Enciclopedia italiana.