Before revision: The four Grand Slam tournaments (also referred to as the majors) are especially popular : the Australian Open, played on hardcourts; the French Open, played on red clay courts; Wimbledon, played on grass courts; and the US Open, also played on hardcourts.
After revision: The four Grand Slam tournaments (also referred to as the majors) are especially popular and are considered the highest level of competition for the sport; these are the Australian Open, played on hardcourts; the French Open, played on red clay courts; Wimbledon, played on grass courts; and the US Open, also played on hardcourts.