I'm not sure if this is THE answer.
But clothes represent the restrictions society puts on people. People dress a certain way based on their station, on the occassion, etc.
So when Atticus loosens his clothing, he is loosening the binds of society and culture on himself.
The reason Atticus loosens his clothes is very simple, the jury is made up of farmers (they probley dont own suits) so to connect with them on a personal level and not seem condisending Atticus loosens and takes off his suit.
Aunt Alexandra moves into Atticus's house in chapter 12.
I think it's Chapter 9
In chapter 10 Atticus and Miss Maudie Atkinson mention that "It's a sin to kill a mockingbird" and that plays into the title of the book.
In chapter 10 Atticus and Miss Maudie mention that "It is a sin to kill a mockingbird." It plays into the title of the book, and also the theme of the book.
Jack is Atticus Finch's brother, (chapter 9)
he says it is "a sin to kill a mockingbird" on page 99 in chapter 10 :)