A dog named Jazzy waits in line with Delta Air Lines passengers at a ticket counter in Newark Liberty International Airport in Newark, N.J., on Aug. 8, 2016. The airline announced new rules Jan. 19, 2018, dealing with animals flying with passengers as service or emotional-support animals.
Seth Wenig, AP

As airlines tweak their policies on bringing a pet on a flight, flyers traveling with their animals this summer must navigate a slew of rules that differ by carrier. But most of what you need to research are three C’s: cabin vs. cargo, crates and cost.

Besides taking Fido or Fluffy to your veterinarian for a preflight checkup, be sure to learn about:

Cabin vs. cargo

If your pet in its carrier can fit under the seat in front of you, it can ride in the airplane cabin for a fee, essentially as carry-on luggage, according to many U.S. airlines’ rules. That includes pets weighing about 20 pounds or less. You can’t buy an extra seat for a bigger pet.

The other way to take your pet — and the only way for bigger animals — is as shipping cargo, such as checked luggage.

Airlines have been flying pets as cargo for years. Even so, the Humane Society of the United States advises against pets being kept anywhere but in the cabin for safety reasons. And some airlines will not transport animals as cargo. Southwest Airlines and JetBlue Airways, for example, offer only in-cabin flights for small dogs and cats.

Not all animals — not even all dogs and cats — are eligible to fly. For example, after a series of well-publicized pet mishaps, United Airlines recently modified its cargo pet-transport program to exclude some breeds of dogs and cats prone to health problems when flying.

More: DOT seeks public comment about how to regulate animals flying with passengers

More: United works with American Humane to improve pet transport; will accept only dogs and cats resuming in July

More: Flying can be dangerous for your pets. Could this help ensure their safety?

A dog is loaded into a charter plane on Aug. 22, 2017 to get airlifted to U.S. in San Juan, Puerto Rico. Various organizations have flown dogs out of Puerto Rico to the mainland in recent years but this latest effort is believed to be the largest in a single trip.
Ricardo Arduengo, AP

Crates

Airlines have detailed requirements for the box your pet rides in, often called a crate, carrier or kennel. Generally, it must be large enough for your pet to stand up and turn around inside.

Airlines and animal experts say to reduce stress, it’s important to acclimate your pet to the crate before flying.

Cost

Air travel with a pet usually involves a pet fee, which can be hefty. Several of the biggest domestic airlines charge $125 each way for an in-cabin pet. And because your in-cabin pet often counts as your carry-on bag, you might have to pay to check your roller bag.

Flying a pet as cargo often costs more; for example, you’ll pay $200 each way on American Airlines when pets are checked as luggage. On United, shipping costs with its PetSafe program are based on the combined weight of the pet and its crate, which can be hundreds of dollars.

Additional expenses include the cost of a pet carrier and a recommended preflight veterinary visit.

Requirements for flying internationally with your pet are more complex and typically require even more planning.

Service dogs and emotional support animals aren’t subject to the same costs and requirements as pets. Contact your airline for details on how to fly with these kinds of animals.

More from NerdWallet:

Guides for flying with dogs or cats

Avoid checked bag fees with an airline-branded credit card

Gregory Karp is a writer at NerdWallet. Email: gkarp@nerdwallet.com. Twitter: @spendingsmart. NerdWallet is a USA TODAY content partner providing general news, commentary and coverage from around the web. Its content is produced independently of USA TODAY.