Americans are separate and limit from one another on many fronts. A fence keeps people out of an area where one does not want anybody going. On the other hand, a fence limits people from coming into contact what is beyond the fence. To have contact with the object outside the fence takes more effort. Morrison uses the two characters to illustrate the race factor in the separations.
People separate themselves by race.
Tywla and Roberta are separated by race, but brought together by circumstance. The two girls happen to be roommates and very much alike. However, as they become older they become very different. The race factor becomes a major issue with the two. The girl's first encounter with each other as adults Roberta shuns Tywla.