Part (a) is a redrawn portion of John Snow’s map on which red dots denote the locations of four pumps. The relative numbers of fatal cases of cholera in each household in the area are represented as blue bars extending at right-angles to the outside of each street. The majority of fatal cases recorded were on Broad Street; these were predominantly in households nearest the pump and also in adjoining streets (e.g. Marshall Street and Berwick Street) and along the nearby section of Silver Street, which runs parallel to Broad Street. In contrast, there was no such clustering of cholera cases around any of the other three pump locations.