The S&P 500 tracks about 500 leading US companies across major industries. Its float-adjusted market-cap weighting gives larger companies more influence. It is widely used as a benchmark for the US large-cap stock market.
History and development
The modern, 500-company S&P 500 debuted on March 4, 1957. It expanded earlier US stock-index work into a broader benchmark of large American businesses across industries.
Its companies are not a permanent roster. Replacements reflect corporate growth, mergers and changes in the economy, so a continuous index history does not mean an unchanged portfolio. The index also predates the ETFs that track it.
References: S&P: index history
Comparing charts on this board
Place the S&P 500 beside the Nasdaq-100 and Dow Jones to distinguish broad large-cap performance from differently weighted groups of companies. Nasdaq-100 outperformance highlights the relative strength of its large non-financial constituents. Even when all three rise, their returns need not match because their membership and weighting differ.
An S&P 500 gain does not mean every constituent rose. Large companies can lift the index while other members decline. Add individual large-cap charts to look for differences in direction rather than treating the headline index as a verdict on every stock. Compare percentage changes, not the indices' raw point levels.
Related charts: NASDAQ 100 · Dow Jones 30 · VTI
Official references: S&P 500