In American law, the clear statement rule is a guideline for statutory construction, instructing courts to not interpret a statute in a way that will have particular consequences unless the statute makes unmistakably clear its intent to achieve that result. According to law professor William Popkin, such rules "insist that a particular result can be achieved only if the text…says so in no uncertain terms."
Protecting constitutional structure
Clear statement rules are commonly applied in areas implicating the structural constitution, such as federalism, sovereign immunity, nondelegation, preemption, or federal spending with strings attached. This is especially true when there is a strong interest against implicit abridgment of traditional understandings.
Congress can abrogate the states' sovereign immunity in some situations. However, it cannot do so implicitly: it must "mak[e] its intention unmistakably clear in the language of the statute."