In Walk Two Moons, why is the plaster wall in Bybanks, Kentucky important to the story?

The plaster wall is an analogy that helps the reader understand the structure of the story, as well as the central theme, which is the search for, and acceptance of, the truth. Salamanca says,

"My father started chipping away at a plaster wall in the living room of our house in Bybanks shortly after my mother left us one April morning...Each night as he waited to hear from my mother, he chipped away at that wall...On the night that we got the bad news - that she was not returning - he pounded and pounded on that wall with a chisel and a hammer...Hidden behind the wall was a brick fireplace."

Sal's father's act of chipping away at the plaster is representative of his search for the truth of what was happening in Sal's mother's life. Once he hears that she is not going to be returning, he has discovered the truth, as shown by the revelation of the brick fireplace underneath all the plaster.

Sal's finding and acceptance of the truth is more complicated than her father's. She at first is not sure in her mind what has happened with her mother, and is unable to even entertain the idea that she might be dead. It is only through following the path of her mother's journey, "walk(ing) two moons" in her moccasins, so to speak, and through the telling of her friend Phoebe's story, that Sal is finally able to accept the reality of what has occurred. Sal needs Phoebe's story to help her face the fact that her mother went away for reasons of her own, and will not be coming back. She observes rightly that

"The reason Phoebe's story reminds me of that plaster wall and the hidden fireplace is that beneath Phoebe's story was another one. Mine" (Chapter 1).

At the end of the story, Sal extends the analogy a step further, developing the theme of the interrelationship of individuals and the need to explore our understanding of them. She says,

"Lately, I've been wondering if there might be something hidden behind the fireplace, because just as the fireplace was behind the plaster wall and my mother's story was behind Phoebe's, I think there was a third story behind Phoebe's and my mother's, and that was about Gram and Gramps."

By "walk(ing) two moons" in her grandparents' moccasins through the act of taking a trip with them, Sal realizes that they have a story worth knowing as well (Chapter 44).