An Ordnung is a district-level agreement, renewed at the members' meeting before each communion, and each of the roughly forty Amish affiliations keeps its own. Because it is oral it can be changed quietly and can also drift, which is why two settlements a hundred miles apart may differ on tractors, indoor plumbing, or whether a phone shanty may stand at the end of the lane. The threat behind it is not force but the ban, and the ban is only credible in a community small enough for everyone to see who is under it. In Wisconsin v. Yoder in 1972 the United States Supreme Court held that Amish parents could not be compelled to send their children to school past the eighth grade, on the grounds that further schooling would endanger the community's religious life. That ruling is the legal foundation of the Amish schoolhouse.