In the United States, a grandfather clause is an exception which allows something pre-existing to remain as it is, despite a change to the contrary in the rules applied to newer situations. It is often used as the verb "to grandfather" or "grandfather in," alternatively, as "grandfather clause." Often, such a provision is used as a compromise, to effect new rules without upsetting a well-established physical or political situation.

Some examples:

• The 22nd Amendment to the Constitution specified term limits for future presidents, but did not apply to the president (Truman) in office when Congress passed it.
• Fire sprinklers are required in all new buildings – but due to the great expense of having older ones retrofitted, they are generally exempt unless and until they are renovated. Such an exception proved deadly to 100 people in 2003 at The Station nightclub fire in Rhode Island, and gutted a large part of an older high-rise office building in the Peachtree 25th fire in Atlanta in 1989.
• Zoning laws often grandfather in existing buildings or other uses, such as when an area is rezoned from residential to commercial, and the existing home on the lot need not be torn down or converted.
• Existing toll roads were allowed to become part of the Interstate highway system in the U.S., even though no new Interstates could (at the time) have tolls.
• Early Internet RFCs which were de facto standards were grandfathered into the official IETF Internet standard process.
• The FCC has required all radio stations licensed in the United States since the 1930s to have four-letter callsigns starting with a W (for stations east of the Mississippi River) or a K (for stations west of the Mississippi River). However, stations with three-letter callsigns and stations west of the Mississippi River starting with a W (plus KDKA, KQV and KYW in Pennsylvania) licensed before the 1930s have been permitted to keep their callsigns.

The source of the term grandfather clause was the laws used from 1895 to 1910 in seven of the southern U.S. states as a Jim Crow law, in order to prevent blacks from voting. In 1870, the Fifteenth Amendment, granting former slaves the right to vote, was ratified. In response, the states passed laws providing that all persons allowed to vote before the American Civil War, and any of their descendants, were exempt from poll taxes levied and/or supposed "literacy" tests required at the time. These laws had the effect of disenfranchising blacks, but not whites, until the ratification of the Twenty-fourth Amendment to the United States Constitution, the 1965 Voting Rights Act, and a 1966 Supreme Court ruling eliminated most legal barriers to black voting.