Standard & Poors S&P500 Equity Index $SPX

The S&P500 equity index is the second only to the DJIA in terms of popularity. The S&P500 is Standard and Poor's index of the largest 500 stocks in the United States. Unlike the DJIA, this index is a value weighted index. The market capitalization of each stock is taken into account when calculating the index. This means that if company A is twice as big as company B then a 1% move in company A will have twice as much effect on the change in the index as a 1% move in company B's stock price.