 Impossibility results in judgment aggregation have the following generic form: for a given class of agendas, the aggregation rules satisfying a particular set of conditions (usually, a domain condition, a rationality condition, and some responsiveness conditions) are non-existent or degenerate (e.g., dictatorial). Different kinds of agendas trigger different instances of this scheme, with stronger or weaker conditions on the aggregation rule leading to an impossibility depending on the properties of the agenda. The significance of combinatorial properties of the agenda was first discovered by Nehring and Puppe (2002, Other Internet Resources) in a mathematically related but interpretationally distinct framework (strategy-proof social choice over so-called property spaces). Three kinds of agenda stand out: