The article explores how countries can distribute emissions permits to regions or entities within their borders for a trading system that helps reduce greenhouse gas emissions economically. It discusses two ways of distributing these permits: giving them based on historical emissions (grandfathering) or auctioning them off. The paper suggests that while some rules for emissions trading should be the same globally, like monitoring and enforcement, each country should decide how to share permits within its borders. It argues that not every aspect of these trading systems needs to be the same across countries, and that governments should have the freedom to choose how to divvy up emissions permits.