The Standard and Poor's 500, or simply the S&P 500, is a free-float weighted measurement stock market index of 500 of the largest companies listed on stock exchanges in the United States. It is one of the most commonly followed equity indices.The S&P 500 index is a capitalization-weighted index and the 10 largest companies in the index account for 27.5% of the market capitalization of the index. The 10 largest companies in the index, in order of weighting, are Apple Inc., Microsoft, Amazon.com, Facebook, Alphabet Inc. (class A & C), Tesla, Inc., Berkshire Hathaway, JPMorgan Chase & Co., and Johnson & Johnson. For a list of the components of the index, see List of S&P 500 companies. The components that have increased their dividends in 25 consecutive years are known as the S&P 500 Dividend Aristocrats.Index funds that track the S&P 500 have been recommended as investments by Warren Buffett, Burton Malkiel, and John C. Bogle for investors with long time horizons.Although the index includes only companies listed in the United States, companies in the index derive on average only 72% of their revenue in the United States.The index is one of the factors in computation of the Conference Board Leading Economic Index, used to forecast the direction of the economy.The index is associated with many ticker symbols, including: ^GSPC, INX, and $SPX, depending on market or website. The index value is updated every 15 seconds, or 1,559 times per trading day, with price updates disseminated by Reuters.The S&P 500 is maintained by S&P Dow Jones Indices, a joint venture majority-owned by S&P Global, and its components are selected by a committee.