Rime of the Ancient Mariner by Samuel Taylor Coleridge is a part of the collection Lyrical Ballads (1798), which he wrote together with William Wordsworth.
A “ballad” is typically a poem/verse that narrates a story, and can be sung because of its rhyming pattern. The ballad scheme was quite popular amongst many Romantic poets.
Coleridge’s Rime of the Ancient Mariner is a good example of literary ballad (the ballad tradition in literature, similar to the folk ballads). It is also a lyrical ballad (an innovation by Coleridge and Wordsworth that put together two earlier-distinct poetic genres), as the narrator tells his personal feelings while narrating the story. One of the simplest similarities is the use of simple and natural characters.
In the poem, a mariner or sailor narrates about a certain journey he made with other sailors into the sea, wherein some supernatural elements occurred on the ship, and hence, like a traditional ballad, there is a lot of action and drama in the story. Additionally, supernatural elements, like ghosts, were typical of early ballads.
Typically a ballad has four lines in one stanza with a rhyming pattern. In Rime of the Ancient Mariner, most of the stanzas have four lines, though we occasionally see more than 4 lines also (reaching 6 lines sometimes), maybe, because Coleridge doesn’t seem too keen to make a compromise on this to maintain this perfect ballad structure.
Coleridge’s Rime of the Ancient Mariner, like a ballad, has iambic tetrameter (4 syllables) in the first and third line, as well as iambic trimester (3 syllables) in the second and fourth line of the stanza. But again, there are exceptions to this.
As mentioned earlier, a ballad can be sung because of its rhyming scheme. Rime of the Ancient Mariner follows a rhyming scheme ABCB, i.e. second and fourth lines rhyme. Besides the rhyming, we also see repetition and alliteration in the poem, much used literary techniques in a ballad.
Water, water, everywhere,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink
About, about, in reel and rout
The death-fires danced at night;
The water, like a witch's oils,
Burnt green, and blue and white.
Though Rime of the Ancient Mariner doesn't strictly adhere to the perfect ballad structure, these slight technical manipulations at places, have created beautiful and miraculous effects, which adds to Coleridge's achievement.