Poetry is one of the most ancient and widespread of the arts. Originally fused with music in song, poetry gained independent existence in ancient times-in the Western world, as early as the classical era (6th century to 4th century BC). Where poetry exists apart from music, it has substituted the lost musical rhythms with its own purely linguistic ones. It is this rhythmic use of language that most easily distinguishes poetry from imaginative prose, the other great division of literature, and that causes poetry to be referred to as metrical writings.
This interpretation does not, however, include cadenced poetry (as in the Bible) or free verse; both of these types of verse are rhythmic but not strictly metrical. Nor does it take into account the strictly oral songs of many past and present cultures. It is, however, a useful starting point for considering what is commonly meant by the word poetry. Poetry generally projects emotionally and sensuously charged human experience in metrical language.
Meter, the highly regular component of verse rhythm, depends basically on the relative strength and weakness of the stresses of adjacent syllables and monosyllabic words. Not all languages have marked differences in syllabic emphasis, however; nor do all poets choose to exploit these differences to create rhythmic patterns. In many languages, poetic rhythm depends more on line length than on differences between syllables.
The line length is traditionally determined by the total number of syllables in a line (syllabic verse), as in French, Italian, Chinese, Japanese, and Welsh poetry; or by the number of stressed syllables in a line (accentual verse), as in Old English alliterative poetry; or by some combination of number and stress, as in the foot verse that became widely used in English poetry beginning in the time of medieval poet.