Partisanship entails a series of related choices. At two analytically distinct but related moments, people decide whether to support a political party or not (partisan support) and which party to name (partisan preference or choice). In Germany, most people say that they incline towards a political party, either the Social Democrats or the Christian Democrats/Socials, and in Britain, most report that they support either Labour or the Conservatives. At a single point in time, selecting a party entails its complement: not naming another party; these are mutually exclusive choices. In both countries, these are rather straightforward outcomes.

Over time, however, choosing between the two parties is more complex. Always naming Party A implies never choosing Party B or any other party, but never choosing B does not entail always naming A or any other party. Sometimes choosing A also implies nothing about the selection of other parties: persons vary in the extent to which they name one of the political parties, and they vary too in the extent to which they move between naming a party and announcing that they prefer no party. Observed over an extended period of time, partisan constancy (the rate of partisan preference) in both Germany and Britain displays large numbers of persons who never support Party A/B and variation along a scale of frequency of choice for Party B/A. Very few move between A and B. Most people behave as if they construct a choice set from the competing political parties, excluding one of the major parties from the frame and then deciding each time whether or not to prefer its major rival.