It is weighted by free-float market capitalization, so more valuable companies account for relatively more weight in the index. The index includes about 80 percent of the American equity market by capitalization.
It comprises 503 common stocks which are issued by 500 large-cap companies traded on American stock exchanges (including the 30 companies that compose the Dow Jones Industrial Average). The S&P 500 stock market index is maintained by S&P Dow Jones Indices.
