I'm not sure I know what you mean by an externality being "covered" by property rights. An externality arises when the social cost or benefit is different from

I'm not sure I know what you mean by an externality being "covered" by property rights. An externality arises when the social cost or benefit is different from the private cost of an action. I assume you are talking about having the actor internalize the costs of the externality by expanding the scope of his property rights. However, I think the parent post was just referring to a result of the Coase theorem. Namely that, no matter the initial allocation of property rights, externalities can be traded away, as long as there are no transaction costs. In practice, negotiating away externalities is questionable for other reasons too, like contract credibility. E.g, if I were to pay someone in front of me on a plane not to recline their seat, they might just pocket the cash and recline anyway.