One of the themes for 2017 has been a weak U.S. dollar. After a post-election rally, the greenback reached multiyear highs, finally peaking in January. Since then, the buck has been on a mostly downward trajectory.

However, the dollar's fortunes may be about to change. The U.S. Dollar Index finally showed some strength in September after a series of consecutive losing months. Could it be the start of a trend?

If the dollar does continue to strengthen, which companies stand to benefit?

Here's a rule of thumb: When the dollar is weak, look outward and invest in companies that do a significant amount of business outside the U.S. Think of some of this year's big winners, like Alphabet (GOOGL) , 3M (MMM) and McDonald's (MCD) . Those companies have significant overseas operations.

However, when the dollar is strong, look inward and buy companies that do the lion's share of their business within the U.S.

A good example would be regional banks. Unlike their multinational counterparts, regional banks have little to no international exposure, and therefore are insulated from the negative effects of a strong dollar. Names like SunTrust Banks (STI) , BB&T (BBT) and Capital One (COF) fall into this category.

What about restaurants? You can find a McDonald's almost anywhere in the world, and that makes it a great stock to own when the dollar is weak. When the dollar is strong, look for chains that have little to no overseas exposure, like Texas Roadhouse (TXRH) , Red Robin Gourmet Burgers  (RRGB) and Sonic (SONC) .

Who else benefits from a strong dollar? We can invest in foreign-based companies that do a significant amount of business inside the U.S. Some companies that fit this description include consumer-products giants Unilever (UL) and Nestle (NSRGF) and commercial airline manufacturer Airbus (EADSY) .

In Washington, proposed changes to corporate tax laws could have a side effect of making the dollar even stronger. White House economic advisor Gary Cohn is floating a proposed 10% tax on funds held overseas by U.S. companies.

Companies like Apple (AAPL) , Alphabet, Cisco Systems (CSCO) and others hold an estimated $2.6 trillion overseas. If the tax rate on funds repatriated to the U.S. is temporarily cut to 10%, it could unleash a significant flow of capital, which in turn would cause an upward surge in the dollar.

The bottom line: 2017 will likely be remembered as the year of the weak dollar, but 2018 could be an entirely different story. Now might be a good time to formulate your game plan.

(This commentary originally appeared on Real Money Pro at 7:00 a.m. on Oct. 13. Click here to learn about this dynamic market information service for active traders.)

Apple, Alphabet is a holding in Jim Cramer's Action Alerts PLUS Charitable Trust Portfolio. Want to be alerted before Cramer buys or sells AAPL and GOOGL? Learn more now.

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At the time of publication, Ponsi was long AAPL, GOOGL, MCD, TXRH and SONC, although positions may change at any time.

Action Alerts PLUS, which Jim Cramer co-manages as a charitable trust, is long AAPL and GOOGL.