The empirical “cube law” applies to parliamentary elections in Anglo-Saxon countries. It says that the ratio of assembly seats of two major parties is approximately the cube of the ratio of votes. This paper presents a more general semi-empirical “seat-vote equation” which includes the cube law as a special case but which also applies to the U.S. Electoral College, labor union, direct presidential, and proportional representation elections. The paper defines “constituency” as the smallest unit within which the party with plurality wins all the seats. The smaller the number of such constituencies is, the more dramatic is the attrition of minority party representation. Thus changes in the number of constituencies can be used to bring about a desired degree of minority representation. The prediction of the average long-range effects of such changes could be an important practical application of the seat-vote equation.