In-cabin carrier
The soft or rigid bag that travels under the seat in front of you. Each airline sets its own maximum dimensions and weight (dog plus bag), so the number that matters is your strictest airline's.
Buy for the strictest airline you plan to fly, not the average one. Three numbers change from airline to airline: the maximum dimensions, the maximum weight, which usually counts the dog plus the bag together, and whether the bag has to be soft-sided. A soft carrier with a flexible frame gets through more counters than a rigid one of the same size, because it compresses under the seat.
Guides that use this term
Gear & stores
In-cabin dog carriers: how to pick one the airline will actually accept
There is no universal cabin carrier size: each airline sets its own dimensions and weight cap. How to buy once and pass the strictest counter.