What code is Atticus talking about during his closing statement?

The "code" that Atticus refers to is the system of racial etiquette and laws that existed in the South from Reconstruction until the 1960s. Known as "Jim Crow," it ensured that whites would remain in a position of power by establishing certain norms and boundaries between the races. Atticus says that Mayella feels guilty for breaking this "rigid and time-honored code" by attempting to have sex with Tom. Having been caught in violation of the code, she responds by lashing out at Tom, blaming him for all that has happened, and accusing him of rape. Interracial sex, especially between black men and white women, was a strong taboo in the South, one which was enforced by law and by extralegal lynchings of the black men who were often portrayed as predators. Mayella played into this belief on the part of many white Southerners, and Tom was put on trial for his life. As Atticus says, "[N]o code mattered to her before she broke it, but it came crashing down on her afterwards" (207). Out of guilt, she shifted the burden of breaking the code onto Tom.