Recent studies have questioned the familiar characterization of Congress as unidimensional. We argue that agenda control, orchestrated through the House Rules Committee and other techniques, can make multidimensional congresses appear more unidimensional. We evaluate this argument by examining the relationship between measures of unidimensionality and various measures of party control for the House of Representatives from 1875 to 1997, at both the roll-call level and congress level. Our findings contribute to an expanding literature explaining why a single dimension could explain most of the variance in voting data, even if latent ideology is multidimensional.