The contending armies were composed of boys. And the boys were residents, respectively, of the Hill and the Valley; two villages, united under the original name of Chestnut Hill, and so closely joined together that it would have been impossible for a stranger to tell where one ended and the other began. The Hill, back on the plateau, had the advantage of age and the prestige that wealth gives. The Valley, established down on the river bank when the railroad was built through, had the benefit of youth and the virtue of aggressiveness. Yet they were mutually interdependent. One could not have prospered without the aid of the other. When the new graded-school building was erected, it was located on the brow of the hill in order to accommodate pupils from both villages. From that time the boys who lived on the hill were called Hilltops, and those who lived in the valley were called Riverbeds. Just when the trouble began, or what was the specific cause of it, no one seemed exactly to know. Like Topsy, it simply grew. With the first snow of the winter came the first physical clash between the opposing forces of Hilltops and Riverbeds. It was a mild enough encounter, but it served to whet the appetites of the young combatants for more serious warfare. Miss Grey, the principal of the school, was troubled and apprehensive. She had encouraged a friendly rivalry between the two sets of boys in matters of intellectual achievement, but she greatly deprecated such a state of hostility as would give rise to harsh feelings or physical violence. She knew that it would be difficult, if not impossible, to coerce them into peace and harmony, so she set about to contrive some method by which the mutual interest of the boys could be aroused and blended toward the accomplishment of a common object.