The first problem in analysing party system change is to acquire a preliminary working definition of what constitutes a party system. From that definition, changes in the elements of a party system may be taken to constitute suggestive evidence of a case of 'party system change'. As a startingpoint it can be suggested that a party system consists of political parties in interaction within a political system. From this statement may be derived three sets of elements which may affect such interaction: 1. the number of parties in a party system; 2. their relative sizes, including the possible existence of 'dominant' par-

ties; 3. their ideological dispersion (or 'spread') along some dimension or dimen-

sions of cleavage, and, related to that, their availability for coalition with each other.