Literary devices are used to convey messages and assist readers in interpreting a text more broadly than they may otherwise do. Similes. metaphor, personification, symbolism, idioms and so on all contribute to the text as a whole. Symbolism is widely used throughout To Kill A Mockingbird, beginning with the title.
Harper Lee uses these literary devices to enrich the meaning of the text and to create interest and to explain some of the events, circumstances and people in a manner which creates a visual picture in the reader's mind. At the beginning of chapter 21, Calpurnia, anxious to find the children, arrives at the courthouse in her "fresh apron." This is a symbol; her apron is, in fact, clean and the symbol has been used to confirm that Calpurnia is always frank and honest. This also indicates very subtly that the court proceedings are certainly not "fresh" or honest by contrast.
Calpurnia is angry at the children and suggests that she will "skin every one of you alive." This well known idiom gives the reader an idea of just how worried about the children she was, only to find them in the courthouse. What she means is that they are in so much trouble and should be punished severely for their actions.
Aunt Alexander's "stroke of paralysis" which Calpurnia suggests she may have if she finds out that the children were in the courthouse means that Aunt Alexander will be so angry and unable to cope or even move if she finds out the children were at the courthouse, and in the "colored" gallery. This metaphor (stroke of paralysis) also reminds the reader that the whole town of Maycomb County lives within a state of paralysis, unable and unwilling to recognize its own prejudices. Suggesting that the upper level of the courthouse is the "colored balcony" also confirms, through use of metaphor, the town's intolerance and hypocrisy. They are in a courthouse, a place of justice, and yet people are unfairly segregated, based on race. The balcony is not colored, the people are and it is ironic and foreshadows the outcome of the trial which will reflect the unfairness of the situation.
There is a play on words when Scout, who does not think that Calpurnia treats the children equally, refers to Calpurnia's "precious Jem." Of course, a "gem"(stone) is a precious diamond. It also confirms that perception is very subjective and Scout, in her childishness, does not see fairness in the same way as she does as an adult.
There are several literary devices in use throughout the chapter, but, moving on to chapter 22, the reader sees personification immediately, in Jem's "angry tears." Tears cannot express emotion and so human characteristics are given to the tears themselves. Scout "stole a glance" at Jem, a metaphor, meaning that she looked secretively at him. There is a very significant simile, and example of irony, when Atticus says to Aunt Alexander, talking about the unfair outcome of the trial, "It's just as much Maycomb County as missionary teas." It is unusual for Atticus to express emotion but his bitterness is obvious and the town's hypocrisy is plainly evident as he is suggesting that prejudice is a usual occurrence, as accepted in the town as the afternoon teas which the missionary circle hold.