Despite challenges from new-fangled upstarts, the good old Standard & Poor's 500 Index still reigns as the king of the stock indexes. It's the basis for such mutual fund giants as Vanguard 500 Index Fund (VFINX) and Fidelity's Spartan 500 Index Fund (FSMKX). In all more than $4 trillion in investor dollars is tracking it (some of which is probably yours). Marketwatch reveals some fascinating facts about the S&P 500 index that you might not know:

• Some people see indexing as a static, sanitized investment strategy. To be sure, the S&P 500 represents about 75% of U.S. stocks by market value, but it's hardly monolithic. Just 86 of the original 500 companies are in the index today. The others were acquired, failed or dropped from the ranks.
• The S&P 500's strength -- ranking stocks by market value -- can be a weakness. In runaway bull markets especially, the index can become a poster child for speculative excesses. When investors ignore valuation and bid shares of the biggest companies to stratospheric heights, the index can become dangerously unbalanced.
• Today, about 18.5% of the S&P 500 is tied to technology and telecom stocks. That's second to financials, at 22% of the index's total value. Add the health-care sector, at 12%, and more than half of the index is represented.
• In the bear market that persisted through most of 2002, index-fund investors found no shelter as the S&P 500 lost half its value.
• To most investors, the S&P 500 is the stock market's apple pie, a uniquely American product. In fact, though the benchmark companies are U.S.-based, their customers are increasingly global. The S&P 500 has so much total international-sales exposure, your stock portfolio might not even need a separate international component for diversification.

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