Dow Jones Industrial Average (DJIA)

The Dow is a price-weighted index of 30 large, established US companies, first published in 1896 and still one of the most quoted market benchmarks despite covering far fewer companies than the S&P 500. Price-weighted means a $500 stock moves the index more than a $50 stock, regardless of which company is actually larger.

That price-weighting is a well-known quirk: a stock split can shrink a component's influence on the Dow overnight even though nothing about the underlying business changed, which is why professional investors generally treat the S&P 500 as the more meaningful gauge of the broad market.