The S&P 500 index ticker symbols were first introduced on March 4, 1957, when Standard & Poor's created the index to track the performance of the stocks of 500 of the most important U.S. companies, according to Reuters. The index's symbols are SPX and INX.
The original S&P 500 index had a total market capitalization of $172 billion and tracked 425 industrial, 15 rail and 60 utility stocks, reports Reuters. Investors use the S&P 500 as the main index against which they measure their portfolios' performance. As of March 2013, the S&P 500 index had a total market cap of $14.6 trillion. Only 69 of the 500 original companies remained in the index.