A constituency in which the distribution of party support is relatively evenly balanced so that the incumbent party has a narrow majority and a small net movement of voters will lead to its changing hands. In many constituencies, the socio‐economic make‐up of the electorate is such as to permanently skew support to one political party, and incumbents have substantial majorities which are normally unassailable by challengers. Such one‐party ‘safe’ seats predominate in many political systems, in which case the outcomes of elections are decided in the ‘marginals’, the often small number of seats in which there are genuine prospects of change in the winning party. For this reason, the parties tend to concentrate their campaigning efforts on wooing voters in those marginal constituencies in which they are expected to come either first or second. It may be noted that seats can, of course, shift between the ‘marginal’ and ‘safe’ categories reflecting population movements, boundary changes, and political realignments.