Golf is played in the United States by about 24 million people. The sport's national governing body, the United States Golf Association (USGA), is jointly responsible with The R&A for setting and administering the rules of golf. The USGA conducts four national championships open to professionals: the U.S. Open, U.S. Women's Open, U.S. Senior Open, and the U.S. Senior Women's Open, with the last of these holding its first edition in 2018. The PGA of America organizes the PGA Championship, Senior PGA Championship and Women's PGA Championship. Three legs of the Grand Slam of Golf are based in the United States: the PGA Championship, U.S. Open and The Masters. (The Open Championship, known in the U.S. as the British Open, is played in the United Kingdom.)