Cut the Cord
In a long time ago, in a galaxy far far away—you know, the 1980s—most houses had only one phone line, and that one phone line had to be connected to a phone with a cord. Teens weren't guaranteed privacy, because their conversations could be listened to by someone picking up another telephone in the house.
The Day family farmhouse has only one phone jack, but Ben, at fifteen years old, wants a phone in his room, so he gets a "line splitter" (2.1) and runs a cord all the way across the house, which splits it. This phone cord divides the house and the family. It makes Patty insecure, because "there had never been long conversations behind closed doors until now" (2.2). What could be going on behind that door? Could it be… Satan?!
Honestly, it's just typical teenage stuff. But family members trip over "that goddang phone cord" (8.19) multiple times during the book, like when Patty goes to cut the lock off Ben's door. Even the curly barbed wire on the prison fence reminds Libby of the goddang phone cord. It was when lines were first drawn, and a minor family war—over nothing at all—grew into a massacre.