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Before this question can be answered, the number of parties must be operationally defined in a way that takes into account their relativesize. Such a number is also needed if one wants to detect trends toward fewer or more numerous parties over time, or the effects of a proposed change in electoral rules. This article presents ways to calculate this important political variable, calculates it for 142 post-1944 elections in 15 West European countries, and analyzes its possible effect on stability. We often talk of two-party and multiparty systems. We further distinguish three-or four-party systems in some countries, and even talk (eg, Blondel, 1969: 535) of a two-and-a-half-party system when there is a third party of marginal size. Mexico could be viewed as a one-and-ahalf-party system because the PRI is so much larger than all other parties. Rather than take the number of all existing parties, including even the very smallest …