The economy has recovered from the depths of the financial crisis, and more folks are gainfully employed. So it’s not exactly a shocker that the Federal Reserve raised its short-term interest rates three times in 2017. But if interest rates are up, then why are mortgage interest rates still hovering around record lows?

It turns out mortgage rates are actually more closely tied to the 10-year U.S. Treasury bond market. Who knew? Bonds are usually considered a safer investment than the more volatile stock market. So when bonds are up, mortgage rates go down.

But the party could be ending. The Fed is expected to raise rates four times in 2018. And mortgage interest rates will likely follow suit and start ticking up.