Games four times during the early 20th century. No international rugby games and union-sponsored club matches were played during the First World War, but competitions continued through service teams such as the New Zealand Army team. During the Second World War no international matches were played by most countries, though Italy, Germany and Romania played a limited number of games, and Cambridge and Oxford continued their annual University Match. The first officially sanctioned international rugby sevens tournament took place in 1973 at Murrayfield, one of Scotland's biggest stadiums, as part of the Scottish Rugby Union centenary celebrations. World Cup and professionalism In 1987 the first Rugby World Cup was held in Australia and New Zealand, and the inaugural winners were New Zealand. The first World Cup Sevens tournament was held at Murrayfield in 1993. Rugby Sevens was introduced into the Commonwealth Games in 1998 and was added to the Olympic Games of 2016. Both men and women's Sevens took place at the 2020 Olympic Games in Tokyo. Rugby union was an amateur sport until the IRB declared the game "open" in August 1995 (shortly after the completion of the 1995 World Cup), removing restrictions on payments to players. However, the pre-1995 period of rugby union was marked by frequent accusations of "shamateurism", including an investigation in Britain by a House of Commons Select committee in early 1995. Following the introduction of professionalism trans-national club competitions were started, with the Heineken Cup in the Northern Hemisphere and Super Rugby in the Southern Hemisphere. The Tri Nations, an annual international tournament involving Australia, New Zealand and South Africa, kicked off in 1996. In 2012, this competition was extended to include Argentina, a country whose impressive performances in international games (especially finishing in third place in the 2007 Rugby World Cup) was deemed to merit inclusion in the competition. As a result of the expansion to four teams, the tournament was renamed The Rugby Championship. Teams and positions Each team starts the match with 15 players on the field and seven or eight substitutes. Players in a team are divided into eight forwards (two more than in rugby league) and seven backs. Forwards The main responsibilities of the forward players are to gain and retain possession of the ball. Forwards play a vital role in tackling and rucking opposing players. Players in these positions are generally bigger and stronger and take part in