High Quality Content by WIKIPEDIA articles! Grandfather clause is a legal term used to describe a situation in which an old rule continues to apply to some existing situations, while a new rule will apply to all future situations. It is often used as a verb: to grandfather means to grant such an exemption. Frequently, the exemption is limited; it may extend for a set period of 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 those rules would apply if the plant were expanded. Often, such a provision is used as a compromise, to effect new rules without upsetting a well-established logistical or political situation. This extends the idea of a rule not being retroactively applied.