We illuminate the role that political geography plays in determining districting outcomes. We find that the political geographic features of a state population limit the types of maps that districting authorities are able to draw. When Democrats are highly segregated – for example, in populous urban areas – it is easier for Republicans to draw very efficient gerrymanders and difficult for Democrats to draw plans that give Democrats an advantage. When we estimate the relationship between Democratic clustering in cities and Republican bias, we only see a correlation in the maps that were drawn by Republicans. This correlation does not occur in maps drawn by Democrats, or by independent actors such as courts and citizen commissions. In sum, political geography only leads to a Republican advantage when Republicans are drawing the lines. This underscores an obvious truth about redistricting: the maps do not draw themselves. Rather, humans choose the maps that best serve their personal and political interests.