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it is the s&p 500
(S)tandard & (P)oor's 500.
The S&P 500 is a market value weighted index of 500 blue-chip stocks, considered to be a benchmark of the overall stock market.
If the S&P 500 is up, usually the market as a whole is also up.
the s means standard and the p is poor. the s and p 500 was to see who had a standard amount of something, the poor people could not use this.
500 squares of paper in a roll?
500 Sheets of Paper (in a Ream)