On Monday, the difference between the 10-year Treasury yield, at 2.97 percent, and the 2-year yield, at 2.82 percent, dramatically narrowed by 5 basis points, the biggest one day move since late March.
Traders have been watching the difference between the yields on various Treasurys for months, along what is called the yield curve between the longer and shorter-term bonds.
And in this time, the longer duration 10-year yield has gotten closer and closer to the yield on the 2-year.
If the two should flip, and the 2 -year yield actually rises above the benchmark 10-year, that inversion would be a signal of a recession.
The two yields are currently just under 15 basis points apart, the narrowest since around the time they last inverted in June 2007.
What's worrisome for some is that on Monday, the difference between the yields on the 3-year and 5-year, and those of the 2-year and 5-year, inverted.