Unpredictable is the norm for bonds right now.

Consider the record of the last couple of years. Bond returns suffered in 2018 as interest rates rose, and many experts bet that rates would stay at lofty levels.

But in 2019, the bond market has defied expectations: Rates have plunged for much of the year, while bond returns have soared.

With interest rates already low, further big gains are improbable. But bonds are still likely to deliver on their crucial role as portfolio stabilizers when stocks fall. Expect protection, not big profits.

It has been easy to overlook bonds’ calming properties lately.

In recent months, they have gotten attention as profit generators, not as havens of stability. Core bond portfolios such as the Vanguard Total Bond Market Index fund and the iShares Core U.S. Aggregate bond E.T.F. gained more than 12 percent from early last November through the first week of September, a period when the yield on the 10-year Treasury note fell to below 1.5 percent from above 3 percent.