The phrase "Jekyll and Hyde" employs a literary reference to describe a stock market that appears to have a split personality, mixing good and bad character traits.

Jekyll represents the good in a market. It is benign, predictable, and conducive to trading gains. Hyde is a bad character who is volatile, unstable, unpredictable, and generally dangerous to investors.

Because the stock market is susceptible to the full range of human emotions, Jekyll and Hyde make frequent appearances on Wall Street.