The euro traded below $1.19 at times on Monday, the lowest level in nine years (it was $1.39 as recently as May). The dollar index, which tracks the dollar against six other major alternatives, is up almost 15 percent since June 30. Those swings are big enough to reshape the terms of economic interaction among the most powerful nations on earth — and will affect almost any company or individual doing business across national borders.

The underlying causes are straightforward enough. The United States economy is doing far better than most other advanced countries, achieving a recently revised 5 percent G.D.P. growth in the third quarter. The Federal Reserve is planning to raise interest rates this year, at a time its counterparts in Europe and Japan are pushing toward easier money.

But the consequences are quite a bit more varied, some straight from Econ 101 and others unique to the current circumstances.

For American companies that compete globally and derive much of their income from overseas, things have gotten a lot tougher in the last few months.

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