The S&P/ASX 20 index is a stock market index of stocks listed on the Australian Securities Exchange from Standard & Poor's. While the "ASX 20" often simply refers to the 20 largest companies by market capitalisation, the S&P/ASX 20 Index is calculated by using the S&P Dow Jones Indices market capitalization weighted and float-adjusted methodologies. All 20 companies also feature in the S&P/ASX 50. The S&P/ASX20 companies are headquartered in four Australian cities with nine headquartered in Melbourne, six headquartered in Sydney, four headquartered in Perth and one headquartered in Adelaide. There are no ASX20 companies headquartered in Australia's third largest city of Brisbane. Wikipedia