The Treasury wants to use short-term borrowing to finance the national debt. While short-term interest rates are usually lower than long-term rates, they’re also more volatile.

On Wednesday, the Treasury Department's decision to double its purchases of longer-dated bonds sent yields lower and gave stocks some breathing room.

The US Treasury moved on Wednesday to calm a bond market rattled by surging borrowing costs, doubling the size of its debt buybacks hours before separate figures confirmed that…