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 , or to have been grandfathered in . 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 allow new rules to be enacted without upsetting a well-established logistical or political situation. This extends the idea of a rule not being retroactively applied. Another common example of a grandfather clause is a customer of a utility or telecommunications service who pays a certain rate and maintains that rate even if other subscribers and new subscribers face higher or increasing rates. If it is an unlimited clause it continues indefinitely and is only cancelled if the subscriber discontinues the services, fails to pay their bill(s) or makes certain changes to their contract.
The term originated in late nineteenth-century legislation and constitutional amendments passed by a number of U.S. Southern states, which created new requirements for literacy tests, payment of poll taxes, and/or residency and property restrictions to register to vote. States in some cases exempted those whose ancestors ( grandfathers) had the right to vote before the Civil War, or as of a particular date, from such requirements. The intent and effect of such rules was to prevent poor and illiterate African-American former slaves and their descendants from voting, but without denying poor and illiterate whites the right to vote. Although these original grandfather clauses were eventually ruled unconstitutional, the terms grandfather clause and grandfather have been adapted to other uses.
The original grandfather clauses were contained in new state constitutions and Jim Crow laws passed by white-dominated state legislatures from 1890 to 1908 in most of the Southern United States to restrict voter rolls and effectively prevent blacks, Mexican Americans (in Texas), and poor whites from voting.  Prohibitions on freedmen's voting in place before 1870 were nullified by the Fifteenth Amendment.
After Democrats took control of state legislatures again after the Compromise of 1877, they began to work to restrict the ability of blacks to vote. Paramilitary groups such as the White League, Red Shirts, and rifle clubs had intimidated blacks or barred them from the polls in numerous elections before the Redemption. Nonetheless, a coalition of Populists and Republicans in fusion tickets in the 1880s and 1890s gained some seats and won some governor positions. To prevent such coalitions in the future, the Democrats wanted to exclude freedmen and other blacks from voting; in some states they also restricted poor whites to avoid biracial coalitions.
White Democrats developed statutes and passed new constitutions creating restrictive voter registration rules. Examples included imposition of poll taxes and residency and literacy tests. An exemption to such requirements was made for all persons allowed to vote before the American Civil War, and any of their descendants. The term grandfather clause arose from the fact that the laws tied the then-current generation's voting rights to those of their grandfathers. According to Black's Law Dictionary , some Southern states adopted constitutional provisions exempting from the literacy requirements descendants of those who fought in the army or navy of the United States or of the Confederate States during a time of war.
After the U.S. Supreme Court found such provisions unconstitutional in Guinn v. United States (1915), states were forced to stop using the grandfather clauses to provide exemption to literacy tests. Without the grandfather clauses, tens of thousands of poor Southern whites were disenfranchised in the early 20th century. As decades passed, Southern states tended to expand the franchise for poor whites, but most blacks could not vote until after passage of the 1965 Voting Rights Act. Ratification in 1964 of the Twenty-fourth Amendment to the United States Constitution prohibited the use of poll taxes in federal elections, but some states continued to use them in state elections.
The 1965 Voting Rights Act had provisions to protect voter registration and access to elections, with federal enforcement and supervision where necessary. In 1966, the Supreme Court ruled in Harper v. Virginia Board of Elections that poll taxes could not be used in any elections. This secured the franchise for most citizens, and voter registration and turnout climbed dramatically in Southern states.
In spite of its origins, today the term grandfather clause does not retain any pejorative sense when used in unrelated contexts.
There is also a rather different, older type of grandfather clause , perhaps more properly a grandfather principle in which a government blots out transactions of the recent past, usually those of a predecessor government. The modern analogue may be repudiating public debt, but the original was Henry II's principle, preserved in many of his judgments, "Let it be as it was on the day of my grandfather's death", a principle by which he repudiated all the royal grants that had been made in the previous 19 years under King Stephen.
- Most cellular phone carriers including AT&T and Verizon had an unlimited data plan in the past, but these plans were removed. Customers who already had unlimited data plans could continue to have them for as long as they kept the same service. For new subscribers, the unlimited plan was no longer available and they had to select from a limited plan (usually 2–4 GB) with extra charges and fees for going over the limit. Recently Verizon introduced a new unlimited plan with provisions to slow down data speed if the subscriber used more than 22GB of data and the network was congested enough to warrant traffic shaping, a practice very common throughout the industry.
- Tablet computers (and similar devices) with screen sizes below eight inches which ran Windows 8 or Windows 8.1, featured a traditional Windows desktop with legacy app support, alongside the ability to run the newer Metro-style apps, but newer devices with sub-8-inch screens that run the newly introduced Windows 10 operating system, no longer have legacy app capability, and as such, can only run apps downloaded only from the Windows Store. However, most (if not all) owners of the older Windows 8 devices of that screen size class who upgrade to Windows 10, can still retain the old-style desktop and legacy app support, despite this upgrade. 
- Many jurisdictions prohibit ex post facto laws, and grandfather clauses can be used to prevent a law from having retroactive effects.
- The United States Constitution appears on a cursory reading to stipulate that presidential candidates must be natural born citizens of the U.S.A. However, there is a further category of persons eligible for that office, now exhausted: those who were citizens of the U.S.A. at the time of the adoption of that constitution. Without that provision, it would have required a strained reading to construe that all actual presidents born in the colonial era were born in the U.S.A., because the United States did not exist prior to July 4, 1776, the date on which the Declaration of Independence was adopted.
- Many acts requiring registration to practice a particular profession incorporated transition or "grandfather sections" allowing those who had already practised for a specified time (often three or four years) to be registered under the act even if they did not have the training or qualifications required for new applications for registration. Examples are the Nurses Registration Act 1901 in New Zealand and the Nurses Registration Act 1919 in Britain.
- In 1949, standards were passed requiring certain fire-safety improvements in schools. However, older schools, such as the Our Lady of the Angels School in Illinois, were not required to be retrofitted to meet the requirements, leading to the deadly Our Lady of the Angels school fire in which 92 students and 3 teachers died.
- In 1952, the United States ratified the Twenty-second Amendment to the United States Constitution, preventing presidents from running for a third term (or a second term, if they had served more than two years of another's term). The text of the amendment specifically excluded the sitting president from its provisions, thus making Harry Truman eligible to run for president in 1952—and, theoretically, for every subsequent presidential election thereafter—even though he had served a full term and almost four years of a previous president's term. Truman was highly unpopular and lost the New Hampshire primary by nearly 55% to 44%. Eighteen days later the president announced he would not seek a second full term.
- In the 1980s, as states in America were increasing the permitted age of drinking to 21 years, many people who were under 21 but of legal drinking age before the change were still permitted to purchase and drink alcoholic beverages. Similar conditions applied when New Jersey and certain counties in New York raised tobacco purchase ages from 18 to 19 years in the early 2000s.
- In 2012, Macau increased the permitted age of entering casinos to 21. However, casino employees between the ages of 18 and 21 before the change were still permitted to enter their places of employment.
- During the Federal Assault Weapons Ban, certain firearms made before the ban's enactment were legal to own. Automatic weapons that were manufactured and registered before the Firearm Owners Protection Act (enacted May 19, 1986) may legally be transferred to civilians.
- According to the Interstate Highway Act, private businesses are not allowed at rest areas along interstates. However, private businesses that began operations before January 1, 1960, were allowed to continue operation indefinitely.
- Michigan law MCL 287.1101–1123 forbade ownership or acquisition of large and dangerous exotic carnivores as pets. But animals already owned as pets at the time of enactment were grandfathered in, and permitted to be kept. 
- The FCC stated that, as of March 1, 2007, all televisions must be equipped with digital tuners, but stores that had TV sets with analog tuners only could continue to sell analog-tuner TV sets.
- In 1967, the FCC prohibited companies from owning both a radio and a television station in the same marketing area, but those already owned before the ruling were permanently grandfathered. For example, ABC already owned WABC-TV, 77 WABC and WABC-FM (now WPLJ), and so could continue to own all three stations after the law was passed. But then-current broadcasting companies that had a radio station in a city could not acquire an adjacent television station, and companies that owned a television station in a city could not acquire adjacent radio stations. In 1996, the law was overturned. Companies can now own up to eight radio stations and two television stations in a market, provided that they do not receive more than 33% of that market's advertising revenues.
- In 1984 Mississippi passed a law changing its official mode of capital punishment from the gas chamber to lethal injection. Under the new law, anyone sentenced after July 1, 1984, was to be executed by lethal injection; those condemned before that date were “grandfathered” into the gas chamber. Therefore, three more convicted murderers would die in the chamber—Edward Earl Johnson and Connie Ray Evans in 1987, and Leo Edwards in 1989. In 1998, the Mississippi Legislature changed the execution law to allow all death row inmates to be executed by lethal injection.
- In 1965, the Canadian government under Prime Minister Lester B. Pearson passed legislation that required senators to retire when they reached the age of 75. However, senators appointed before the legislation was passed were exempted from the mandatory retirement rule. 
- When Quebec enacted the Charter of the French Language in 1977, making French the province's official language, the famous Montreal kosher-style deli Schwartz's was allowed to keep its name with the apostrophe, a feature not used in French in the possessive form.
- During Canada's federal Redistribution, a grandfather clause ensures that no Province can have fewer seats after Redistribution than it did in 1985. 
- In the early 2000s the Houston Police Department mandated to police academy graduating classes that tenured officers are required to carry a .40 caliber sidearm - tenured police officers prior to the mandate were grandfathered in where they still carried their existing sidearms.
- In 2013, Tennessee enacted a law requiring that products labeled as " Tennessee whiskey " be produced in the state, meet the legal definition of bourbon whiskey, and also use the Lincoln County Process. The law specifically allowed Benjamin Prichard's Tennessee Whiskey, which does not use the Lincoln County Process, to continue to be labeled as such.   
- In 2014, Kentucky radically simplified its classification of cities, with the previous system of six population-based classes being replaced by a two-class system based solely on the type of government effective January 1, 2015. In the old classification system, many cities had special privileges (notably in alcoholic beverage control, taxing powers, and certain labor laws) based on their class; the new legislation contained elaborate provisions to ensure that no city lost a privilege due to the reclassification. 
- Strict building codes to withstand frequent seismic activity were implemented in Japan in 1981. These codes applied only to new buildings, and existing buildings were not required to upgrade to meet the codes. One result of this was that during the great Kobe earthquake, many of the pre-1981 buildings were destroyed or written off, whereas most buildings built post-1981, in accordance with the new building codes, withstood the earthquake without structural damage.
- Wigwag -style railroad crossing signals were deemed inadequate in 1949 and new installations were banned in the United States. Existing wigwag signals were allowed to remain and 65 years later, there are still about 40 wigwag signals in use on railroads in the USA.
- The UK's national rail network Network Rail requires new locomotives and rolling stock to pass tests for Electromagnetic compatibility (EMC) to ensure that they do not interfere with signalling equipment. Some old diesel locomotives, which have been in service for many years without causing such interference, are exempted from EMC tests and are said to have acquired grandfather rights.
- The Steel Electric-class ferryboats used by Washington State Ferries were in violation of several Coast Guard regulations, but because they were built in 1927, before the enactment of the regulations, they were allowed to sail. Those ferries were decommissioned in 2008.
- Tolled highways that existed before the Interstate Highway System are exempt from Interstate standards despite being designated as Interstate highways. Many such toll roads (particularly the Pennsylvania Turnpike) remain as such. However, tolled highways built since the Interstate system, such as the tolled section of PA Route 60 and PA Turnpike 576, must be built or upgraded to Interstate standards before receiving Interstate designation. Both highways are to be part of the Interstate system, with PA 60 now I-376 and PA Turnpike 576 to become I-576 in the near future. As well, U.S. Interstate Highway standards mandate a minimum 11-foot median; however, highways built before those standards have been grandfathered into the system. The Kansas Turnpike is the most notable example, as it has been retrofitted with a Jersey barrier along its entire 236-mile length.
- The earliest Ontario 400-series highways and other expressways do not meet current standards, however it would be prohibitively expensive to immediately rebuild them all to updated guidelines, unless a reconstruction is warranted by safety concerns and traffic levels. As a result, substandard sections of freeways such as low overpasses and short acceleration/deceleration lanes are often retrofitted with guard rail, warning signage, lower speed limits, or lighting.
- The United States Federal Communications Commission has required all radio stations licensed in the United States since the 1930s to have four-letter call signs starting with a W (for stations east of the Mississippi River) or a K (for stations west of the Mississippi River). But stations with three-letter call signs and stations west of the Mississippi River starting with a W and east of the Mississippi River starting with a K—such as WRR in Dallas and WHB in Kansas City, plus KQV and KYW in Pennsylvania, all licensed before the 1930s—have been permitted to keep their call signs. In the western United States, KOA in Denver, KGA in Spokane, KEX in Portland, and KHJ and KFI in Los Angeles, among many others, have been permitted to keep their original or reassigned three-letter call signs. In addition, a new or existing station may adopt a three-letter call set if they have a sister radio or TV station in that market with those calls (examples include WJZ-FM Baltimore and WGY-FM Albany, New York).
- In aviation, grandfather rights refers to the control that airlines exert over “slots” (that is, times allotted for access to runways). While the trend in airport management has been to reassert control over these slots, many airlines are able to retain their traditional rights based on current licences.
- In the UK, until 1992, holders of ordinary car driving licences were allowed to drive buses of any size, provided that the use was not commercial and that there was no element of "hire or reward" in the vehicles' use; in other words, no one was paying to be carried. The law was changed in 1992 so that all drivers of large buses had to hold a PCV (PSV) licence, but anyone who had driven large buses could apply for grandfather rights to carry on doing so. 
- Some MOT test standards in the UK do not apply to vehicles first registered prior to the implementation of the legislation that introduced them. For example, vehicles first registered prior to January 1, 1973 are exempt from the requirement to use retro-reflective Yellow/White vehicle registration plates and vehicles first registered prior to January 1, 1965 are exempt from Seat Belt standards/legislation unless they have been retrospectively fitted.  
- In some U.S. states, the inspection/maintenance (I/M) programs for motor vehicle emission testing have a rolling chassis exemption, e.g. a motor vehicle model 25 years old or more is exempted from emission tests.
- The National Football League (NFL) currently prohibits corporate ownership of teams. Ownership groups can have no more than 24 members, and at least one partner must hold a 30% ownership stake. The league has exempted the Green Bay Packers from this rule; the team has been owned by a publicly owned, nonprofit corporation since 1923, decades before the league's current ownership rules were put in place.
- Similarly, in association football the Bundesliga prohibits corporate ownership of more than 49% of teams and rebranding as well. So does Royal Dutch Football Association with Eredivisie. However, Bayer Leverkusen and PSV Eindhoven can maintain their property and names as they were founded in the first decades of the twentieth century by Bayer and Philips as their sport teams.
- Beginning in 1979, the National Hockey League required all players to wear helmets. But if a player had signed his first professional contract before this ruling, he was allowed to play without a helmet if he so desired. Craig MacTavish was the last player to do so, playing without a helmet up until his retirement in 1997, other notable players include Guy Lafleur and Rod Langway who retired in 1991 and 1993, respectively.  Kerry Fraser was the last referee who was not required to wear a helmet, until the ratification of the new NHL Officials Association collective bargaining agreement on March 21, 2006.
- Three former venues in the National Hockey League — Chicago Stadium, Boston Garden and Buffalo Memorial Auditorium —had shorter-than-regulation ice surface, as their construction predated the regulation. The distance was taken out of the neutral zone. All three arenas were replaced by newer facilities by 1996. The regulation does not apply in many minor league venues, and in older minor league venues shorter than regulation, the distance was taken from neutral zones.
- Six schools that are members of NCAA Division III, a classification whose members are generally not allowed to offer athletic scholarships, are specifically allowed to award scholarships in one or two sports, with at most one for each sex. Each of these schools had a men's team that participated in the NCAA University Division, the predecessor to today's Division I, before the NCAA adopted its current three-division setup in 1973. (The NCAA did not award national championships in women's sports until 1980–81 in Division II and Division III, and 1981–82 in Division I.) Two other schools were formerly grandfathered, but have moved their Division I sports to Division III.
- In 2006, NASCAR passed a rule that required teams to field no more than four cars. Since Roush Racing had five cars, they could continue to field five cars until the end of 2009.
- Jackie Robinson's #42, which he wore when he broke Major League Baseball's 20th-century color line and throughout his Hall of Fame career with the Brooklyn Dodgers, is the subject of two such grandfather clauses.
- In 1997, MLB prohibited all teams from issuing #42 in the future; current players wearing #42 were allowed to continue to do so. New York Yankees' closer Mariano Rivera was the last active player to be grandfathered in, wearing #42 until he retired after the 2013 season.  However, since 2009, all uniformed personnel (players, managers, coaches, umpires) are required to wear #42 (without names) on Jackie Robinson Day.
- In 2014, Robinson's alma mater of UCLA, where he played four sports from 1939 to 1941, retired #42 across its entire athletic program. (The men's basketball team had previously retired the number for Walt Hazzard.) Three athletes who were wearing the number at the time (in women's soccer, softball, and football) were allowed to continue wearing the number for the rest of their UCLA careers. 
- In 1920, when Major League Baseball introduced the prohibition of the spitball, the league recognized that some professional pitchers had nearly built their careers on using the spitball. The league made an exception for 17 named players, who were permitted to throw spitballs for the rest of their careers. Burleigh Grimes threw the last legal spitball in 1934.
- The NFL outlawed the one-bar facemask for the 2004 season but allowed existing users to continue to wear them (even though by that time, the mask had been mostly out of use, save for a handful of kickers/punters). Scott Player was the last player to wear the one-bar facemask.
- The NFL introduced a numbering system for the 1973 season, requiring players to be numbered by position. Players who played in the NFL in 1972 and earlier were allowed to keep their old numbers, although New York Giants linebacker Brad Van Pelt wore number 10 despite entering the league in 1973 (Linebackers had to be numbered in the 50s at the time; since 1984 they may now wear numbers in the 50s or 90s. Van Pelt got away with it because he was the team's backup kicker his rookie season).  The last player to be covered by the grandfather clause was Julius Adams, a defensive end ( 1971 – 1985, 1987) for the New England Patriots, who wore number 85 through the 1985 season. He wore a different number during a brief return two years later.
- Major League Baseball rule 1.16 requires players who were not in the major leagues before 1983 to wear a batting helmet with at least one earflap. The last player to wear a flapless helmet was the Florida Marlins' Tim Raines in 2002 (career began in 1979). The last player eligible to do so was Julio Franco in 2007 (career began in 1982), although he opted to use the flapped version.
- For many decades, American League (AL) umpires working behind home plate used large, balloon-style chest protectors worn outside the shirt or coat, while their counterparts in the National League wore chest protectors inside the shirt or coat, more akin to those worn by catchers. In 1977, the AL ruled that all umpires entering the league that year and in the future had to wear the inside protector, although umpires already in the league who were using the outside protector could continue to do so. The last umpire to regularly wear the outside protector was Jerry Neudecker, who retired after the 1985 season. (Since 2000, Major League Baseball has used the same umpire crews for both leagues.)
- The National Hot Rod Association is enforcing a grandfather clause banning energy drink sponsors from entering the sport if they were not sponsoring cars as of April 24, 2008, pursuant to the five-year extension of its sponsorship with Coca-Cola, which is changing the title sponsorship from Powerade to Full Throttle Energy Drink.
- Even though tobacco advertising in car racing was banned, the Marlboro cigarette brand, owned by British American Tobacco in Canada, and Philip Morris International elsewhere, is grandfathered in to sponsoring a car in the F1 series on the agreement that the name is not shown in places that banned it.
- NASCAR allows some grandfathered sponsorships by energy drink brands in the top-level Monster Energy Cup Series, while rules prohibit new sponsors in that category. Similar policies with regard to telecommunications companies were in effect when the series was sponsored by Sprint. Additionally, some insurance company sponsorships were grandfathered in when the second-level series now known as the Xfinity Series was sponsored by Nationwide Insurance.
- In August 2014, the Baseball Hall of Fame and the Baseball Writers' Association of America (BBWAA) announced changes to the Hall of Fame balloting process effective with the election for the Hall's induction class of 2015. The most significant change was reducing the time frame of eligibility for recently retired players from 15 years to 10. Three players on the 2015 BBWAA ballot who had appeared on more than 10 previous ballots— Don Mattingly, Lee Smith, and Alan Trammell —were exempted from this change, and remained eligible for 15 years (provided they received enough votes to stay on the ballot). 
- In November 2015, Little League Baseball changed its age determination date from April 30 to August 31—a calendar date that falls after the completion of all of the organization's World Series tournaments—effective with the 2018 season. The rule was written so that players born between May 1 and August 31, 2005, who would otherwise have been denied their 12-year-old season in the flagship Little League division, would be counted as 12-year-olds in the 2018 season. 
- In December 2016, French Rugby Federation (FFR) president Bernard Laporte announced that all future members of France national teams in rugby union and rugby sevens would be required to hold French passports. At the time, eligibility rules of World Rugby, the sport's international governing body, required only three years' residency for national team eligibility, and did not require citizenship. Players who had represented France prior to the FFR policy change remain eligible for national team selection. 
In certain scenarios "to grandfather" a system, component or interface involves running multiple versions of that system, component or interface. In the case of Webservice/SOAP calls, this is done by webservices versioning, where the current wsdl interface (grandfather) runs alongside the new webservice.