The fear gauge: how it works

Stocks go up and down. They boom, then bust, then boom.

But what if they didn't? What if the market, instead of an up-and-down slog, was a steady, ever-upward saunter? For more than a year, that's how it looked.

From late 2016 through the end of last month, the market was cool, calm and collected, with the Standard and Poor's 500-stock index climbing more than 30 percent while a key measure of stock-market volatility - the VIX, also known as the "fear gauge" - edged ever lower.

There has been an abrupt reversal in past few weeks, with big swings in the equity markets that have focused