> each airline has its own rules exactly - "has rules". Very few actually enforce them. And while I don't know whether the aircraft industry agreed on some sta

> each airline has its own rules exactly - "has rules". Very few actually enforce them. And while I don't know whether the aircraft industry agreed on some standard, my pretty huge, hard-case backpack always perfectly fits in. And most airlines have actually the same rules how large items may be to qualify as carry-on: Worldwide you see the same measuring cages at check-in counters or the gate and trolleys are advertised as "carry on compatible" - both would not be possible without a maybe unwritten but pretty well established standard. Edit: And you of course are right about very small commuter planes - but they are, IMHO not really relevant in the discussion because they are used less and less due to economic disadvantages of making something airborne for just 50 people.