"Harlem" is a lyric poem with irregular rhyme and an irregular metrical pattern that sums up the white oppression of blacks in America. It first appeared in 1951 in a collection of Hughes's poetry, Montage of a Dream Deferred.

Theme

Frustration

In 1951–the year of the poem's publication–frustration characterized the mood of American blacks. The Civil War in the previous century had liberated them from slavery, and federal laws had granted them the right to vote, the right to own property, and so on. However, continuing prejudice against blacks, as well as laws passed since the Civil War, relegated them to second-class citizenship. Consequently, blacks had to attend poorly equipped segregated schools and settle for menial jobs as porters, ditch-diggers, servants, shoeshine boys, and so on. In many states, blacks could not use the same public facilities as whites, including restrooms, restaurants, theaters, and parks. Access to other facilities, such as buses, required them to take a back seat, literally, to whites. By the mid-Twentieth Century, their frustration with inferior status became a powder keg, and the fuse was burning. Hughes well understood what the future held, as he indicates in the last line of the poem. 

 

Meter, Structure, Rhyme, and Technique

Although the meter of "Harlem" varies, the poem has a rhythmic, lyrical quality achieved through alliteration, rhyme, repetition of certain words, and carefully placed stressed syllables. The length of the first five lines also varies: Line 1 has eight syllables, Line 2 has four, Line 3 has seven, Line 4 has six, and Line 5 has three. This irregularity gives these lines a jagged edge, like the edge of a shard of broken glass, enabling Hughes's message to lacerate its readers. However, the last three lines of the poem each have five syllables, smoothing the poem's edge to the keenness of a razor ready to cut cleanly. Although the poem does not imitate any format used by previous poets, it does exhibit regularities, including the following:    

In each line except Line 7, the last syllable is stressed.
Six of the seven sentences in the poem are questions.
All of the sentences except the first and the last contain similes using like.
An alliterating d appears in the first two lines, and an alliterating s in Lines 3, 4, 6, 7, 8, and 9.
Line 3 rhymes with Line 5; Line 6 rhymes with Line 8; Line 10 rhymes with Line 11. 
Lines 4, 7, and 11 begin with or.
Lines 3, 8, and 10 begin with like.

Alliteration, Simile

Hughes relies primarily on alliteration (as noted above, under Meter, Structure, Rhyme, and Technique) and simile (a comparison that uses like, as, or than to reveal similarities between unlike things). Similes using like compare the result of the dream deferred of Line 1 to other results: the drying up of a raisin, the festering of a sore, the stinking of rotten meat, the crusting of a piece of uneaten candy, and the sagging of a heavy load.  

Anaphora/Repetend

Hughes also uses anaphora, or repetend, from Line 2 onward. Anaphora is a figure of speech in which a word or phrase is repeated at the beginning of a clause or another group of words. Anaphora imparts emphasis and balance. Repeated words are does it (Lines 2, 6, and 11) and like a (Lines 3, 4, 6, 8, and 10).

Graphic Sights and Smells

The language of "Harlem" is frank, down-to-earth. It does not euphemize. The narrator asks whether a dream becomes a dried-up fruit, a running sore, rotten meat, or a sweet that crusts and sugars over. He also asks whether the dream sags or explodes. All of these tropes  enable to reader to see and smell the frustration of American blacks. 

Maltreatment of Blacks Since the Civil War

The 13th, 14th, and 15th Amendments to the U.S. Constitution–approved in the post-Civil War era–granted black Americans basic rights as American citizens, as did the Civil Rights Act of 1875. However, court and legislative decisions later emasculated the legal protection of blacks. For example, the U.S. Supreme Court ruled in 1896 (Plessy v. Ferguson) that it was legal to provide "separate but equal" accommodations for passengers of Louisiana's railroads. This ruling set a precedent that led to segregated schools, restaurants, parks, libraries, and so on. Meanwhile, hate groups inflicted inhuman treatment on innocent blacks, including brutal beatings. Lynchings of innocent blacks were not uncommon. Many so-called "enlightened" or "liberal-minded" Americans looked the other way, including law-enforcement officers, clergymen, politicians, and ordinary Americans. By the mid-20th Century, black frustration with white oppression formed itself into a potent blasting powder.   
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