The keystone of this story is most simply put, the black box. The box is described as a small, rugged antique (older than Mr. Warner even). Jackson describes the box as it "grew shabbier each year: by now it was no longer completely black but splintered badly along one side to show the original wood color, and in some places faded or stained." Not only does the box hold the tickets to the lottery within it, but it holds the tradition of the lottery itself. Mr. Summers is noted in the story to have suggested creating a new box, however, the villagers repeatedly rejected the idea. This is symbolic of the change from archaic ideas, such as human sacrifice to a newer, modern tradition, perhaps one that doesn't require someone dying at the end.
While the tradition may still be archaic in its ideals, characters such as Mr. Warner give the reader insight that it has still indeed changed. The box is also symbolic of this change, as its appearance has withered away with time. Now it is no longer even a completely black box, but a stained and damaged one, much as the ritual itself is no longer its original self, stained and damaged in ways of its own. Parallels from this box can be seen all throughout the story, which is why I feel it is important to note it as a centerpiece of the story.