Queen Elizabeth II is the longest-reigning British monarch since her great-great-grandmother, Queen Victoria. During her time on the throne Her Majesty has met the sitting U.S. president at least once since the 1950's. </br></br>Britain's Queen Elizabeth II and President Donald Trump inspect a Guard of Honor, formed of the Coldstream Guards at Windsor Castle, July 13, 2018, in Windsor, England. Her Majesty welcomed the President and first lady at the dais in the quadrangle of the Castle. Getty Images

President Barack Obama looks on, as Queen Elizabeth II signs a guest book, after a reciprocal dinner at the Winfield House in London, May 25, 2011. </br></br>The Obamas enjoyed a regal welcome from Queen Elizabeth II. The second stop on his European tour, comes as Britain seeks to prove its staying power despite fading military might and Washington looks to retool its decades-old alliance with Europe as a catalyst for global action. Jewel Samad/AFP/Getty Images

President George W. Bush was welcomed to the United Kingdom by Queen Elizabeth II and Prime Minister Tony Blair, Nov. 19, 2003, amid tight security at Buckingham Palace. </br></br>Traditional British pomp and ceremony, included a 41-gun salute, at the start of the historic state visit. Kirsty Wigglesworth/AFP/Getty Images

First lady Hillary Rodham Clinton, left, President Bill Clinton, Queen Elizabeth II, and Chelsea Clinton, right, stand for photographers outside Buckingham Palace in London, Dec. 14, 2000. </br></br>Clinton met with the queen on the final day of his three-day visit to Ireland, Northern Ireland and England. Ron Edmonds/AP

President George Bush and Queen Elizabeth II shake hands with Mike Gallego, Rick Honeycutt, and Jose Canseco of the Oakland Athletics at game between the Athletics and the Baltimore Orioles in 1991. Wally McNamee/Corbis

Queen Elizabeth II presents President Ronald Reagan with an honorary knighthood, June 14, 1989, at Buckingham Palace in London, alongside first lady Nancy Reagan and His Majesty, Prince Phillip. </br></br>Reagan was the first U.S. president in more than 35 years to be knighted by the queen, of which only Republican presidents have received the honor. David Levenson/Getty Images

When President Jimmy Carter met Queen Elizabeth II in London's Buckingham Palace on May 1, 1977, he declined to bow and instead went for a kiss. </br></br>The queen was said to have held a grudge against Carter over the hello smooch for years. AP

President Gerald Ford not only met Queen Elizabeth but took her for a spin around the dance floor when she visited the White House in 1976. Just as the two hit the dance floor, as shown here, the Marine band struck up "The Lady is a Tramp." The dance came during a state dinner honoring the queen and Prince Philip at the White House, July 17, 1976, in Washington. Universal History Archive/Getty Images

Queen Elizabeth II and British Prime Minister Edward Heath, left, join President Richard Nixon and first lady Patricia at Chequers, Heath's official country residence, Oct. 3, 1970. President Nixon made at least two trips to Britain during his presidency. </br></br>Nixon's predecessor, Lyndon Johnson, was the only sitting U.S. president that did not meet the queen during her 60-year reign. Hulton Archive/Getty Images

President John F. Kennedy, right, and his wife, First Lady Jacqueline Kennedy, 2-L, pose with Queen Elizabeth, 2-R, and her husband, Prince Philip, Duke of Edinburgh, at Buckingham Palace in London, June 15, 1961. Photoquest/Getty Images

Queen Elizabeth II standsoin the grounds of Balmoral Castle in Scotland alonside President Eisenhower, Aug. 29, 1959. </br></br>Prince Philip and Princess Anne joined the President and the Queen for the photo op on the castle grounds. AP

President Harry S. Truman and Britain's Princess Elizabeth are shown as their motorcade got underway following the reception ceremony at Washington National Airport today. </br></br>The President welcomed Queen Elizabeth and her husband, Prince Philip, Duke of Edinburgh, with the warm hope that "when you leave you will like us even better than when you came." Bettmann/Corbis