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Is the S&P 500 index’s recent rally real, or is it just a bear-market bounce?

That’s always a question investors have when the market is rising after a significant selloff. Given all the uncertainty regarding tariffs — as U.S.-China trade talks kick off on Saturday — and worries about the economy, that question becomes even more important.

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While the S&P 500’s SPX selloff, from its record close of 6,144.15 on Feb. 19 to its one-year closing low of 4,982.77 on April 9, didn’t quite meet the bear-market criteria of a 20% decline — it only fell 18.9% — it was certainly significant.

On Friday, the index closed 13.6% above the recent low, which is also a rally significant enough for investors to question whether a new uptrend has started.

So how do chart watchers answer that question? They use math — and nature.

The math is based on a number sequence made famous by the 13th-century Italian mathematician known as Fibonacci of Pisa, in which you get the next number by adding up the previous two. It goes: 1, 1, 2, 3, 5, 8, 13, 21, and so on.

The longer the sequence goes, the closer the ratio of the latest two numbers gets to the Fibonacci ratio of 0.618.

This is where nature comes into play, because Fibonacci found the pattern to be prevalent in natural systems — including sea shells, the breeding patterns of rabbits, a galaxy’s spiral and the human body.

Because many on Wall Street see the behavior of financial markets as an extension of their living participants, naturally, chart watchers adopted the Fibonacci ratio as a guide.

The idea is that if a bounce surpasses 61.8% of the selloff that preceded it, then it’s no longer governed by the selloff, meaning it’s likely a new uptrend. The next upside target becomes a full retracement of the previous downtrend.

For the S&P 500, the 61.8% retracement of its 1,161.38-point drop, on a closing basis, to the April 9 low comes in at 5,700.50.

Since Fibonacci levels offer a point of reference during uncertain times, they often provide resistance on the way up and support on the way down.