Political scientists regularly justify particular democratic institutions. This article explores two desiderata for such justifications. The first is a formal equality baseline which puts the burden of justification on those who favour more unequal institutions. This baseline is seen as an implication of the rule of law. The second desideratum, the comparison requirement, builds on the first: adequate justifications of particular institutions must compare them to the best alternative, and this comparison must consider the costs for political equality. The two desiderata are applied to political science debates about the proportionality of the electoral system and bicameral systems of legislative decision-making.