The concentration of political and financial power in empires kept the number of capital cities small through long stretches of urban history. Planning historians taking a comparative approach have paid attention to the international diffusion of capital city planning ideas; Ward's model of the diffusion of planning differentiates between types of "borrowing" and plans that were forms of "imposition". Berlin exemplifies how planning history contributes to planning practice through networks of ideas that circulate beyond national and language borders. The civic center became a standard American planning element, but similar plans emerged in other countries, where municipal power might be expressed by magnificent city halls or a complex of institutional and cultural buildings. The Garden City model was appealing as the major planning approach when the sponsoring political regime wished to showcase planning for working-class communities like communist Moscow and socialist Dodoma.