This study focuses on the principal rules of government formation as well as on the deviant cases. On a party level, over 1,000 cases and approximately 250 government formations in 17 West European countries during the second half of the twentieth-century are analysed. By means of regression analyses, the study explores the effects of the size of the parties, other party characteristics, as well as characteristics in the party system on the choice of premier party and coalition party respectively. The results show that the choice of premier party to a great extent is decided by the size of the party and the position as median party. The choice of coalition party, however, is a far more complex process. Favourable and unfavourable conditions are defined, and the deviant cases are identified, compared and analysed with respect to the choice of PM party and the choice of coalition party respectively. The study shows that the deviant cases are not disparate. On the contrary, the deviant cases form a set of rules of their own.