The US Treasury is set to auction off a record amount of debt this week, according to the Wall Street Journal 's Tom Lauricella . Defying conventional wisdom, the market for U.S. government debt is rallying thanks to an unusual combination of buyers including American households, banks and the Federal Reserve. The rally has taken Treasury yields -- which move opposite the bonds' price -- to their lowest levels since spring, and have helped push mortgage rates to their lowest levels in three months. The Fed's active presence has also raised questions of whether the rally is sustainable. The appetite for Treasurys, generally considered safe, points to an undercurrent of wariness about the health of the economy long term, even as investors have lately loaded up on riskier investments, like stocks and junk bonds. Typically, stocks and other risky investments move in the opposite direction of Treasurys in the short term. News of this "unlikely rally ," as Lauricella calls it, provides a good opportunity to step back and help people understand the difference between the Treasury bonds, notes, and bills. And Paddy Hirsch provides a nice primer on the latest Marketplace Whiteboard . Take a look: