Border paperwork decides whether your pet may enter a country. The airline decides how it boards — a separate rulebook, run by a different organisation, checked at a different counter. There are exactly three ways onto an aircraft, and size decides most of it.
In the cabin, under the seat
Small cats and dogs — typically under roughly 8 kg carrier included, though every airline sets its own cap — ride in a soft carrier under the seat in front of you. Two catches. The carrier’s dimensions must fit that airline’s specific under-seat measurements, not “a” carrier. And aircraft carry a fixed number of pets per cabin, first come, first served — the pet’s reservation can sell out while seats for humans remain, which makes booking the animal’s place an early step, not an afterthought.
As checked baggage
Mid-size animals once travelled routinely as excess baggage on the same aircraft as their owner. Fewer carriers offer this every year — several large airlines have ended it entirely — so treat it as a route-by-route question rather than an assumption, and confirm before building a plan on it.
The hold — and where we hand over
Larger dogs and long multi-leg routes go in the climate-controlled hold as manifested cargo, in a rigid crate sized to the animal. Doing this properly — crate certification, route planning around temperature embargoes, handling at connections — is a licensed profession with its own body, IPATA. It is the one leg of the journey PetPaperwork refers out by design: we prepare the documents; moving the animal is the specialist’s job.
The breed caution nobody expects
Snub-nosed breeds — bulldogs, pugs, Persian cats and their relatives — face outright airline restrictions for the hold, because their airways handle heat and stress poorly. Many carriers refuse them below deck in any season; some refuse them entirely. If your pet is brachycephalic, the boarding question is the first question of the whole plan, not the last.
The through-line: airline approval and border approval are two separate files. A pet the airline happily boards can still be refused entry on landing, and a pet with a perfect border file can still be denied boarding at check-in. The trip works when both stacks are right — which is why they belong in one plan, on one calendar.