 A modal property \(N\) is called alethic just in case the claim that a proposition has \(N\) entails that the proposition is true. Metaphysical, epistemic and nomic necessity are all alethic. By contrast, moral and legal necessity are not. It is both morally and legally necessary (i.e., it is required both by morality and by the law) that no murders are committed, even though murders are in fact being committed. A modal property defined by restriction from an alethic kind of necessity must itself be alethic. By contrast, relativization allows us to define non-alethic modal properties from alethic ones, by relativizing to a class of propositions that contains some falsehoods. Similarly, we can define a non-alethic modal property from an alethic one by restricting the quantifier over possible worlds to some class that does not include the actual world. For example, legal necessity can perhaps be defined from metaphysical necessity by restricting the quantifier to worlds where everybody conforms to the actual laws.