What kind of poem is La Belle Dame sans Merci by John Keats?

"La Belle Dame sans Merci" is a ballad, an old form of verse adapted for singing or recitation. The ballad form originated in the days when most poetry was memorized rather than written, and the typical subject matter of the ballad reflects a folk sensibility. Ballads are usually narrative, or storytelling, poems, and early ballads often addressed themes important to common people: love, courage, the mysterious, and the supernatural. Though the ballad is generally rich in musical qualities such as rhythm and repetition, it often portrays both characters and events in highly dramatic but simplistic terms.

Additional characteristics of the typical ballad include a set rhyme scheme and alternating line lengths. Formally, the ballad stanza is a quatrain, or a group of four lines, in which the first and third lines contain four stressed syllables while the second and fourth lines contain three stressed syllables. "La Belle Dame sans Merci" consists of twelve such stanzas, with a slight variation: the last line of each stanza contains only two stressed syllables, creating a dramatic suspension between stanzas. Aside from this, the quatrains exhibit the typical ballad stanza pattern of rhyme: the second and fourth lines are set in perfect end rhyme with one another, giving the poem the musical sound most ballads feature.