Poetry, and discussion of it, has a long history. Early attempts to define poetry, such as Aristotle's Poetics, focused on the uses of speech in rhetoric, drama, song and comedy. Later attempts concentrated on features such as repetition and rhyme, and emphasised the aesthetics which distinguish poetry from prose. From the mid-20th century, poetry has sometimes been more loosely defined as a fundamental creative act using language.
Poetry often uses particular forms and conventions to expand the literal meaning of the words, or to evoke emotional or sensual responses. Devices such as assonance, alliteration, onomatopoeia and rhythm are sometimes used to achieve musical or incantatory effects. Poetry's use of ambiguity, symbolism, irony and other stylistic elements of poetic diction often leaves a poem open to multiple interpretations. Similarly, metaphor and simile create a resonance between otherwise disparate images — a layering of meanings, forming connections previously not perceived. Kindred forms of resonance may exist, between individual verses, in their patterns of rhyme or rhythm.
Some forms of poetry are specific to particular cultures and genres, responding to the characteristics of the language in which the poet writes. While readers accustomed to identifying poetry with Dante, Goethe, Mickiewicz and Rumi may think of it as being written in rhyming lines and regular meter, there are traditions, such as those of Du Fu and Beowulf, that use other approaches to achieve rhythm and euphony. In today's globalised world, poets often borrow styles, techniques and forms from diverse cultures and languages.
Poetry as an art form may predate literacy. Many ancient works, from the Vedas (1700–1200 BC) to the Odyssey (800–675 BC), appear to have been composed in poetic form to aid memorization and oral transmission, in prehistoric and ancient societies. Poetry appears among the earliest records of most literate cultures, with poetic fragments found on early monoliths, runestones and stelae.
The oldest surviving poem is the Epic of Gilgamesh, from the 3rd millennium BC in Sumer (in Mesopotamia, now Iraq), which was written in cuneiform script on clay tablets and, later, papyrus. Other ancient epic poetry includes the Greek epics, Iliad and Odyssey, the Roman national epic, Virgil's Aeneid, and the Indian epics, Ramayana and Mahabharata.
The efforts of ancient thinkers to determine what makes poetry distinctive as a form, and what distinguishes good poetry from bad, resulted in "poetics" — the study of the aesthetics of poetry. Some ancient societies, such as the Chinese through the Shi Jing, one of the Five Classics of Confucianism, developed canons of poetic works that had ritual as well as aesthetic importance. More recently, thinkers have struggled to find a definition that could encompass formal differences as great as those between Chaucer's Canterbury Tales and Matsuo Bashō's Oku no Hosomichi, as well as differences in context spanning Tanakh religious poetry, love poetry, and rap.
Context can be critical to poetics and to the development of poetic genres and forms. Poetry that records historic events in epics, such as Gilgamesh or Ferdowsi's Shahnameh, will necessarily be lengthy and narrative, poetry used for liturgical purposes (hymns, psalms, suras and hadiths) is likely to have an inspirational tone, and elegy and tragedy are meant to evoke deep emotional responses. Other contexts include Gregorian chants, formal or diplomatic speech, political rhetoric and invective, light-hearted nursery and nonsense rhymes, and even medical texts.
Classical thinkers employed classification as a way to define and assess the quality of poetry. Notably, the existing fragments of Aristotle's Poetics describe three genres of poetry — the epic, the comic, and the tragic — and develop rules to distinguish the highest-quality poetry in each genre, based on the underlying purposes of the genre. Later aestheticians identified three major genres: epic poetry, lyric poetry and dramatic poetry, treating comedy and tragedy as subgenres of dramatic poetry.
Aristotle's work was influential throughout the Middle East during the Islamic Golden Age, as well as in Europe during the Renaissance. Later poets and aestheticians often distinguished poetry from, and defined it in opposition to, prose, which was generally understood as writing with a proclivity to logical explication and a linear narrative structure.
This does not imply that poetry is illogical or lacks narration, but rather that poetry is an attempt to render the beautiful or sublime without the burden of engaging the logical or narrative thought process. English Romantic poet John Keats termed this escape from logic, "Negative Capability." This "Romantic" approach views form as a key element of successful poetry because form is abstract and distinct from the underlying notional logic. This approach remained influential into the twentieth century.
During this period, there was also substantially more interaction among the various poetic traditions, in part due to the spread of European colonialism and the attendant rise in global trade. In addition to a boom in translation, during the Romantic period numerous ancient works were rediscovered.
One defining characteristic of poetry is meter, or the pattern of stresed and unstressed syllables. These syllables are then grouped into units called "feet". The feet are put together in a line that is named according to how many feet it contains. A poetic scheme is described in terms of both feet and lines. For example, iambic pentameter, used by Shakespeare, consists of lines of five feet, each foot being an iamb:
|Ye elves of hills, brooks, standing lakes and groves,|
|And ye that on the sands with printless foot|
|Do chase the ebbing Neptune, and do fly him|
- The Tempest, Shakespeare, 5.1
In many languages, including modern European languages and Arabic, poets use rhyme in set patterns as a structural element for specific poet forms, such as ballads, sonnets and rhyming couplets. However, the use of structural rhyme is not universal even within the European tradition. Much modern poetry avoids traditional rhyme schemes. Classical Greek and Latin poetry did not use rhyme. Rhyme entered European poetry in the High Middle Ages, in part under the influence of the Arabic language in Al Andalus (modern Spain). Arabic language poets used rhyme extensively from the first development of literary Arabic in the sixth century, as in their long, rhyming qasidas. Some rhyming schemes have become associated with a specific language, culture or period, while other rhyming schemes have achieved use across languages, cultures or time periods. Some forms of poetry carry a consistent and well-defined rhyming scheme, such as the chant royal or the rubaiyat, while other poetic forms have variable rhyme schemes.
Most rhyme schemes are described using letters that correspond to sets of rhymes, so if the first, second and fourth lines of a quatrain rhyme with each other and the third line does not rhyme, the quatrain is said to have an "a-a-b-a" rhyme scheme. This rhyme scheme is the one used, for example, in the rubaiyat form. Similarly, an "a-b-b-a" quatrain (what is known as "enclosed rhyme") is used in such forms as the Petrarchan sonnet.
Some poetry did not rely on rhyme but on other sound devices. Alliteration and assonance played a key role in structuring early Germanic, Norse and Old English forms of poetry. The alliterative patterns of early Germanic poetry interweave meter and alliteration as a key part of their structure, so that the metrical pattern determines when the listener expects instances of alliteration to occur. This can be compared to an ornamental use of alliteration in most Modern European poetry, where alliterative patterns are not formal or carried through full stanzas. Alliteration is particularly useful in languages with less rich rhyming structures. Assonance, where the use of similar vowel sounds within a word rather than similar sounds at the beginning or end of a word, was widely used in skaldic poetry, but goes back to the Homeric epic. Because verbs carry much of the pitch in the English language, assonance can loosely evoke the tonal elements of Chinese poetry and so is useful in translating Chinese poetry. Consonance occurs where a consonant sound is repeated throughout a sentence without putting the sound only at the front of a word. Consonance provokes a more subtle effect than alliteration and so is less useful as a structural element.