Anyone who’s ever shopped for a home loan knows they come in two main flavors: fixed rate and adjustable rate mortgages, or ARMs. While traditional 30-year fixed mortgages have long been a homeowner favorite, sometimes an ARM can be a smart move.

Here’s how ARMs work. For a period of years – usually 3, 5, 7 or 10 – the mortgage behaves like a standard fixed-rate loan. You’ll know your rate upfront and it won’t change during that initial period.

After that, your lender can adjust your rate, raising it if national rates have moved higher, or lowering it if rates have dropped. Therein likes the risk with ARMs since no one can reliably predict where rates will move several years in the future.

Of course, you’ll earn a trade-off in exchange for an ARM’s risk. You’ll notice that ARM rates are noticeably lower than 30-year fixed rates. So while they are less predictable over time, you’ll be guaranteed to pay a lower rate for the initial period.

That means an ARM could be a wise choice if you expect to stay in your home less than the number of years in the ARM’s fixed period.

But if your expectations prove wrong and you live in the home long enough to reach your ARM’s adjustable period, you’ll find yourself at the mercy of current market rates. Right now, rates are forecasted to be on an upswing given the Federal Reserve’s movements. But after that, it’s impossible to know where rates will be headed.

In the end, adjustable rate mortgages are an easy choice when you know you won’t live in your home for the long haul. But if you’re like the many homebuyers who aren’t sure how long they’ll stay, a fixed-rate mortgage can be the safer and more penny-wise move.