A grandfather clause (or grandfather policy) is a provision in which an old rule continues to apply to some existing situations while a new rule will apply to all future cases. Those exempt from the new rule are said to have grandfather rights or acquired rights. Frequently, the exemption is limited; it may extend for a set time, or it may be lost under certain circumstances. For example, a "grandfathered power plant" might be exempt from new, more restrictive pollution laws, but the exception may be revoked and the new rules would apply if the plant were expanded. Often, such a provision is used as a compromise or out of practicality, to effect new rules without upsetting a well-established logistical or political situation. This extends the idea of a rule not being retroactively applied.