But now the driving forces behind them have changed, giving them a whole new importance for Cramer.
For years, bonds mostly moved higher because of inflation and the Federal Reserve. If bond holders thought inflation was coming or that the Fed would raise rates, they dumped bonds and rates went up. Great job growth and big retail sales also drove rates higher.
There were also outlying moments such as times of crisis when investors don't trust other assets, money will flood into risk-free assets like U.S. Treasurys.
Now, President Donald Trump just became a new outlier, Cramer said. When Trump seems to be in authority and talks about his tax plan, and investors start believing that the economy will heat up and sell bonds, he sends interest rates higher.
After all, if Trump gets his way, inflation will surge and money managers will look like idiots for holding bonds. Thus, the sellers are trying to get ahead of that scenario occurring.
Then when Trump starts discussing contentious issues like a travel ban, cabinet appointments, Trump's clothing line being dumped by Nordstrom or a Supreme Court pick; investors get cold feet and sell stocks to buy bonds.
On Thursday, Trump met with airline related executives, and once again Cramer saw the action in bonds play out as the stock market roared higher.
"In other words, the bonds call the tune, not the stocks," Cramer said.
So, Cramer recommended for investors to watch bonds. They have now become a roadmap on Trump's plans for the economy, he said. When bond yields go higher, Cramer expects industrial and the banks to higher.
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