Using 2,500 recorded votes from altogether 30 legislative terms, we assess the determinants of the political dimensionality of German state parliaments (Landtage). We locate MPs and their parties by fitting two-dimensional item-response models. Visual inspection of the voting maps and multivariate tests show that legislative voting is shaped by the conflict between government and opposition, but that the left-right dimension is – even though less pronounced – also present. Furthermore, our results reveal interesting explanations for variation in the dimensionality between parliaments and over time. Most importantly, the type of coalition determines how closely positions in the voting maps correspond with externally measured positions on different scales and influences the political dimensionality as a whole. Whereas grand coalitions strongly overlay the true preference variation of parties and MPs, left- or right-wing governments seem to allow for greater variation. Our analysis of legislative voting in very similar political systems points to the need to study the impact of different coalition types on legislative voting.
