This article tests the hypothesis that parties in West European parliamentary systems operate under the constraint of “policy horizons,” that is, limits or bounds on the extent to which they can compromise on policy for the purpose of entering coalition governments. The test is based on a new expert survey covering thirteen West European parliamentary democracies in which respondents were asked not only to locate party positions on a number of policy dimensions, but also to estimate the parties' limits of acceptable compromise on each dimension. The survey data are first analyzed using a new computer program, Horizons 3D, to determine which parties have intersecting horizons in these systems—and hence the ability to form coalition governments under the hypothesis. These calculations are then employed to assess whether policy horizons structure the choice of governing coalition beyond any effect conveyed by the policy distances among parties. Although the potential for error in these data is considerable, the estimated horizons, with few exceptions, appear to play the role hypothesized for them.