An index fund (also index tracker) is a mutual fund or exchange-traded fund (ETF) designed to follow certain preset rules so that the fund can track a specified basket of underlying investments. While index providers often emphasize that they are for-profit organizations, index providers have the ability to act as "reluctant regulators" when determining which companies are suitable for an index. Those rules may include tracking prominent indexes like the S&P 500 or the Dow Jones Industrial Average or implementation rules, such as tax-management, tracking error minimization, large block trading or patient/flexible trading strategies that allow for greater tracking error but lower market impact costs. Index funds may also have rules that screen for social and sustainable criteria.