In recent months, the U.S. stock market has gone nowhere: up, down, up, down. No pattern.

Yesterday, the world's stock markets tanked. No warning. Wham!

No single central bank is responsible. No single central bank can goose the world's stock markets. No coordinated policy is likely . . . yet.

There is no sell-off panic . . . yet.

This was "a warning shot fired across the bow."

This has the look of a market top.

One day's stick market action rarely calls attention to itself. This one is not comparable to the 22% worldwide one-day collapse in October 1987. But do not shrug this off. See what happens next week. If all stock markets continue to fall, take this price action seriously.

You may not be in the stock market. But stock market declines often herald a recession. That can affect you.