The US Open is the fourth and final grand slam of the year. It is held over a two week period from August into September. The US open has been going on since 1881. It was originally only open to men, but it was called The U.S. National Singles Championship. It was also held at a different location of Newport, Rhode Island in contrast to where it's held today in Queens, New York City. Only club members of the United States National Lawn Tennis Association could enter into the tournament. From 1884 through 1911, there was a challenge system where the defending champion automatically qualified for the next year's final. In 1915, the tournament moved to Forest Hills, New York. From 1921 to 1923 it was played in Philadelphia but returned to Forest Hills in 1924. In 1887 the women had their tournament, but in Philadelphia. When the open era began, the five events (men's singles, women's singles, men's doubles, women's doubles, and mixed doubles) were merged into the US Open. It was, then, also open to professionals. In 1970, it was the first to use a tiebreak at the end of a set. The US Open was originally played on grass until 1975. In 1978 the tournament moved once more to its current location of Queens, New York City. In 2006, the US Open was the first to use the challenge system. Players could challenge a call that they disagree with, and the Hawk Eye computer system could pin-point the exact location of the ball. This could determine whether the call was correct or not.