A granfather clause is an exception that allows an old rule to continue to apply to some existing situations, when a new rule will apply to all future situations. It is often used as a verb: to grandfather means to grant such an exemption. For example, a "grandfathered power plant" might be exempt from newer and tougher pollution laws. 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.