Juxtaposition:
The placement of elements, characters, scenes, objects, etc. side by side for purposes of comparison and contrast.

Example:
"We have men-stealers for ministers, women-whippers for missionaries and cradle-plunderers for church members."
-Narrative of the life of Frederick Douglass

Explanation:
This is a juxtaposition that compares two things that are unlike, but during that time period it was. This leads to one purpose, of comparing the two unlike things so the reader can get a glimpse of what the people were like, and how Douglass saw them. This comparison shows how wrong the people of that time were, and how hypocritical they were. This is the main purpose of Douglass's autobiography, and even more importantly the appendix.