King's primary rhetorical technique in "A Letter from Birmingham Jail," a characteristic of much of his writing, is the highly creative use of metaphorical language to make an abstract concept into a concrete, easily visualized, image in his reader's mind. For example, he uses the phrase "dangerously structured dams" in paragraph 24 to describe the abstract concept of law and order's failure to establish justice. Likewise, he equates the tension created by racial injustice to a "boil that can never be cured as long as it is covered up. . . ," arguing that it must be lanced so that "air and light" (conscience and national opinion) can cure it, not a pretty metaphor, to be sure, but a very appropriate comparison and very visual.
Another characteristic rhetorical device used by King to good effect is anaphora, the repetition of key words and phrases. We see this device in paragraph 25 as King argues that condemning non-violent protests "because they precipitate violence" is like condemning the victim of robbery because he possesses money. King begins three sentences, each of which contains an example supporting his overall argument,w ith the phrase, "Isn't this like condemning. . . ." These balanced sentences, using identical diction (words) and diction (sentence structure), carry King's argument effectively forward because they each contain separate, but identical, examples that validate his argument.
King uses anaphora again in paragraph 26 when repeats the opening phrase he used in paragraph 24--"I had hoped" and "I had also hoped"--an effective technique for unifying thes paragraphs whose subject is his disappointment with the "white moderate." In addition, by using the word "hoped," he is reminding his audience that he is very disappointed at the response from those whom he thought would be his greatest allies, and he is gently reminding them that his hope in their support is almost gone. This is his way of encouraging them to renew their efforts on behalf of the Negro cause without demanding that they do so.