The effects have been mixed: Negative rates on foreign debt have helped keep a wide range of interest rates in the United States unusually low, which translates into good deals for borrowers and bad for ones for lenders, including major banks, whose profit margins have been compressed. For older people trying to live on their bond investments, low rates result in paltry income and painful choices.

The unusual conditions defined by negative interest rates also complicate matters for the Federal Reserve, which has begun to raise short-term rates in the United States, only to see longer-term rates decline. That unexpected development is occurring partly because below-zero rates in European and Japanese bond markets have created greater demand for American bonds, raising their prices and driving down yields, which move in an opposite direction. Negative rates have not been common for very long and their consequences aren’t entirely understood.

Still, grasping the basics of how negative rates work and how they may be affecting the United States is important for anyone with money in the markets.

Consider that if you deposit your savings with a bank that has negative interest rates, the longer the bank holds your money, the less you will have in your account. The flip side of this is marvelous: The longer it takes to repay a negative-interest-rate mortgage, the less you owe, even if you pay nothing. In short, negative rates can make saving money seem foolish, while borrowing can become epically attractive. (The European banks generally have not deliberately passed negative rates on to retail customers, but negative-interest-rate mortgages cropped up temporarily in Denmark for some people lucky enough to have had adjustable rate mortgages.)

Negative rates appeared during the global financial crisis in 2008, when yields on some United States Treasury bills fell below zero for brief periods. That happened because so many panicky people wanted to buy Treasuries that their prices soared and their yields dropped into negative territory. In essence, investors were willing to lose a small amount of principal in exchange for the safety. Those negative interest rates were a sign that the financial world was in trouble.

Today’s negative rates are not a short-term market anomaly. To the contrary, central banks in Europe and Asia have made negative interest rates part of their official tool kit. The European Central Bank; the central banks of Switzerland, Sweden and Denmark; and, last month, the Japanese central bank have all deliberately set at least some short-term rates below zero — effectively penalizing commercial banks for depositing funds with them, and giving them a strong incentive to move their money elsewhere.

Negative rates abroad have caused another problem for the United States, and, perhaps, created an opportunity.