‘One person, one vote’ (OPOV) is an important slogan in democratic movements, a principle that undergirds a landmark series of cases in US constitutional law, and a widely accepted axiom of democratic theory in philosophy and political science. It is taken to be sacrosanct; some even state that OPOV “is, like the injustice of chattel slavery, a ‘fixed point’” (Kolodny 2023: 291). This is a rare distinction for an ideal. For all the ink spilt on Rawls’ Difference Principle, no one (...) printed it on the pamphlets and placards of trade unionist, decolonial, and anti-Apartheid protests; it never appears in US Supreme Court decisions; and none of its defenders would say that denying it is akin to accepting slavery. But despite this, OPOV has proved remarkably hard to pin down. It is not meant to be a mere slogan. It is meant to explain why practices of plural voting and vote dilution are inegalitarian. But there is no consensus and little debate about what it means. Many leave the principle undefined. Others say OPOV means anonymity, or equal voting weight, or equal representation, or equal voting power. These accounts differ significantly. But little is said about one should be preferred to its rival. -/- My goal is to convince you that the meaning of OPOV poses a deep, urgent problem, for which we have no adequate solution. I will argue that all current accounts of OPOV should be rejected. The key upshot will be that we need, but lack, a good explanation of the datum that practices of plural voting and vote dilution robustly fail a minimal condition for political equality. In closing, I’ll explain why my arguments also push us towards embracing what has often been considered a radical and overly demanding implication: namely, that all district-based systems—including those used in the US House of Representatives, Senate, and Electoral College—violate OPOV. (shrink)