July 23 (Reuters) - Focus on hockey at the 2020 Tokyo Olympics: THE ABSOLUTE BASICS * For both men"s and women"s hockey, 12 nations will compete in two groups of six. The top four teams from each group progress to the quarter-finals. * Each team takes the field with 11 players, with rolling substitutions permitted. * Matches last 60 minutes, played over four 15-minute periods. HOW MANY MEDALS? One gold in each tournament. WHAT HAPPENED IN RIO? Argentina became the first South American team to secure Olympic gold in the men"s tournament after their victory over Belgium in the final. In the women"s tournament, Britain dethroned tournament favourites the Netherlands in a shootout to take home their first Olympic gold medal in women"s hockey. WHAT CAN WE EXPECT IN TOKYO? Defending champions Argentina will again be in the mix but Belgium, Australia, Germany, the Netherlands and India are the most likely medal contenders. In the women"s tournament, the Netherlands are favourites as top-ranked team in the world and after clinching silver in 2016 and gold in 2012 and 2008. Britain, champions in 2016, are expected to push them all the way. WHAT"S NEW? Japan"s men, who made their last Olympic appearance at the 1968 Mexico City Games, have qualified as hosts this time around. WHEN IS IT HAPPENING? From July 24 to Aug. 6 WHERE IS IT HAPPENING? At the Oi Hockey Stadium, which was newly constructed in a park near Tokyo Bay. HOW DID WE GET HERE? Hockey became an Olympic sport at the 1908 London Games. It has been at every Olympics since except the 1912 and 1924 Games. The women"s tournament was added at the 1980 Moscow Games. WELL FANCY THAT The matches will mostly be played in the mornings and evenings, reportedly due to concerns about the impact Tokyo"s sweltering summer heat could have on players, as well as clearance for planes to use a flight path over Tokyo to Haneda airport, which sits about 5km away from the hockey venue.