If an airfare seems too good to be true, it probably comes with a catch.

Think of that in early 2017 when you suddenly start seeing United Airlines offer “basic economy” low fares on some routes.

The catch?

You can’t bring a carry-on bag to put in any overhead compartment. Whatever you bring must fit under your seat.

The “basic economy” fare also will not offer seat assignments ahead of time, and will restrict your ability to modify the reservation.

United is not the first airline to dream up this type of fare. In fact, the trend of “unbundling” airline tickets so passengers pay extra for every little thing, from carry-on bags to seat assignments to water, is escalating.

Competitor Delta also offers “basic economy” fares on select routes, but so far still allows those passengers to bring carry-on bags aboard without charge. Spirit Airlines allows its low-fare passengers to bring a carry-on bag aboard — if they pay $35.

While flight attendants will like anything that cuts down on the number of people trying to stuff bulky suitcases into cramped overhead compartments, for travelers it’s another story.

United’s move is a reminder to examine restrictions on fares you see labeled with “basic,” “ultra low” or similar jolly euphemisms. Such a fare might work for you in some circumstances.

Just don’t be blindsided.