Residential segregation cannot occur without social inequality, because a society that is wholly egalitarian in its socioeconomic make-up will contain no groups (classes or strata) that can be differentiated by their geographical location. Conversely, high levels of socioeconomic inequality may be found in both segregated and unsegregated cities. The territorial character of segregation means, however, that people's geographical mobility is a key factor, as it can alter their residential situation and, in the aggregate, change the levels and patterns of residential segregation.