Sam Dunlap on Yahoo Finance: Stocks Rally on Trade Optimism

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Dow: A stock market index that tracks the daily price of shares of stock of 30 large publicly-owned companies sold on the U.S. stock exchanges.

NASDAQ Composite: A market capitalization-weighted index of the common stocks and similar securities listed on the NASDAQ stock exchange. 

S&P 500 Total Return Index: An American stock market index based on the market capitalizations of 500 large companies having common stock listed on the NYSE or NASDAQ.

It is not possible to invest directly in an index.