What fits: cabin rules, inches and litres

A 20-inch case is twenty inches tall including the wheels and the retracted handle. Airlines measure the same way, and American Airlines says so in one line.

That is the sentence to keep in mind when a case is advertised as a 22-inch carry-on. Twenty-two inches of shell plus wheels puts the bag over the line, and the sizer bin at the gate settles it.

Delta publishes two numbers, a limit per side and a limit for the three added together.

Southwest allows a larger bag in the cabin, and the same page names the part travellers forget.

The 24 and 28 inch shells open a zip around the middle and gain about twenty percent of volume. The gain is real and it runs one way: a packed and expanded case will not close back down until you unpack it.

The 20-inch carry-on has no expansion zip. We left it out because the case exists to pass a cabin rule, and a bag that can grow past the rule fails it in the one moment that counts.

Soft bags carry a litre figure because their shape changes with what you put in them. Sixty litres holds a week of clothes for one person, or a weekend for two. Forty litres in backpack form takes a laptop, a change of clothes and a toilet bag, and it lies flat for the scanner.

For the seat in front of you, Delta describes the allowance in its own words. A tote or a cosmetic case belongs in that space. A 40-litre backpack, packed full, usually does not.

The Transportation Security Administration states the rule plainly, and then names the limit per container.

Our wide-open cosmetic cases hold a quart bag upright and open flat on a hotel counter, which is the reason they exist. They are not a sealed case, so anything that can leak travels inside the quart bag first.

A hard shell protects what it surrounds and refuses to change shape. For a bag that has to squeeze into a car boot around other luggage, the soft oxford set gives more and forgives an extra jumper.

For a single overnight stay, none of the cases here are the right answer. A 60-litre duffle weighs about three pounds empty and takes a night's clothes without the wheels, the handle and the eight pounds that come with them.