A rare “death cross” appeared Tuesday in the chart of the Dow Jones Industrial Average, suggesting the stock market may have already begun a new long-term downtrend.

Although chart watchers have seen the bearish technical pattern coming for some time, it can still send a chill down bulls’ spines when it is finally confirmed.

The fact that the Dow industrials’ DJIA, -1.84% death cross follows the appearance of one in its sister index, the Dow Jones Transportation Average DJT, -2.64% , warns that this one is more than a one-off event.

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A death cross is said to have occurred when the 50-day simple moving average, which many use to track the short-term trend, crosses below the 200-day moving average, which is widely used to gauge the health of the longer-term trend. For the Dow industrials, it marked the first time the 50-day moving average, which ended Tuesday at 17,806.99, was below the 200-day moving average, at 17,813.42, since Dec. 30, 2011, according to FactSet.

Many technicians see the death cross as marking the spot that a shorter-term pullback morphs into a longer-term downtrend.

The Dow followed Tuesday’s death cross by falling 1.4% in morning trade Wednesday, putting it on track to suffer its ninth loss in 10 sessions. It has lost 6.3% since its record close of 18,312.39 on May 19.

Some technicians argue that death crosses actually signal buying opportunities during bull markets, since some previous ones have appeared right around market bottoms. Others say it’s not a true death cross unless the 200-day moving average is declining when the 50-day moving average crosses below it.

“Death cross” has worked often enough to scare bulls FactSet

The 50-day moving average (or “MA”) crossed below a rising 200-day MA on July 7, 2010, when the Dow closed at 10,018.28. The Dow’s closing low for 2010 was actually hit two sessions earlier, at 9,686.48.

But the Dow fell another 5.9% over six weeks after the Aug. 24, 2011 death cross, and tumbled as much as 50% over 14 months after the one appearing on Jan. 3, 2008.

And keep in mind that when the January 2008 death cross appeared, the Dow had lost just 7.8% from its Oct. 9, 2007 peak. That means the bull market was still firmly in place, as the rule of thumb is a bear market is defined by a decline of at least 20% from a significant peak. In addition, the 200-day moving average didn’t turn lower until two weeks after the death cross appeared.

On Wednesday, the Dow’s 200-day MA rose by less than two points on the day, after rising by nearly four points on Tuesday and by over five points on Monday.

The following is the death-cross situation for other market benchmarks:

• The S&P 500 Index’s SPX, -1.15% 50-day moving average was at 2,094.67 on Wednesday, while the 200-day MA was at 2,075.27, up by just half a point on the day.

• The NYSE Composite NYA, -2.11% also produced a death cross Tuesday, as the 50-day MA moved below the 200-day MA for the first time since Feb. 26. On Wednesday, the 200-day MA was less than half a point above Tuesday’s value.