"Snow drew upon a centuries-old mathematical tool that would later be termed the Voronoi diagram. (It is unlikely that Snow knew anything of the device's history, though he was certainly the first to apply it to disease mapping.) A Voronoi diagram conventionally takes the shape of a two-dimensional field made up of points surrounded by 'cells.' The cells define the region around each point that is closer to that particular point than any other point in the diagram."
—Steven Johnson, The Ghost Map (New York: Penguin, 2006), 195