Langston Hughes' poem entitled, "Harlem," uses a couple of literary devices—including imagery often in the form of similes. However, "Harlem" does not have a set meter or rhyme scheme.
Meter is the beat or rhythm of in a poem. Meter is defined as:
...a rhythm of accented and unaccented syllables which are organized into patterns, called feet.
"Harlem" does not have a set rhythm because it includes several lines of varying lengths.
Hughes' poem "Harlem" is written in free verse, which does not have a recognizable pattern. The poem is "free" of rules for rhyming or rhythmic patterns. Free verse is poetry that…
...refrains from consistent meter patterns, rhyme, or any other musical pattern...
Langston Hughes does use rhyme in "Harlem," but it does not follow a set configuration. Charting the rhyme (by using the last word of each line) shows no discernible pattern. The pattern of the poem's single "section" is abcbded. The last two lines rhyme, but they are not presented as a rhyming couplet. A rhyming couplet is generally made up of two related lines of text—that rhyme.
Good rhyming couplets tend to "snap" as both the rhyme and the idea come to a quick close.