This chapter presents popular preferences and party policy together to tackle the central supply and demand problem of democratic politics – how do its processes ensure a necessary connection between public policy and popular preferences? It shows how general ideas about the way voters, electors and parties interact can be made more precise by representing them as spatial relationships – that is, as distances between points in a space. ‘Policy inertia’ is the general rule for governments and bureaucracies, only partially overcome by party initiatives. Slow change in actual policy and frequent alternation between governments with different policy targets thus keeps current applied policy broadly in the centre of the policy space, where most citizens prefer it to be. The democratic process produces a dynamic rather than a completely stable policy equilibrium. Inter-election policy proposals trigger a negative ‘thermostatic’ reaction not just on their own demerits but also as part of the government’s general programme for change.