The rule is simple. One bag. Under the seat in front of you or in the overhead bin. It does not get checked. It does not grow. It does not multiply into a personal item plus a carry-on plus the jacket you're holding because there's no more room. One bag. That's the whole rule.
The one-bag discipline starts with a question most travelers never ask: what do I actually use? Not what might I need, not what if the weather changes, not what if there's a formal dinner I hadn't planned for. What do I actually use. The answer, for almost every trip in almost every climate, is less than you think. A lot less.
The bag itself is not the point. The bag is a constraint that forces clarity. When you cannot bring more, you bring only what matters. You stop treating your suitcase as a contingency plan and start treating the trip as the thing you are actually optimizing for. The bag becomes invisible. The destination becomes everything.
The one-bag community has been working this out for twenty years on forums and blogs and comment sections. The collective knowledge is significant: which packing cubes compress most reliably, which fabrics dry overnight, which shoes work in every context, which toiletries can be bought anywhere. The infrastructure for one-bag travel exists. All that's required is the decision to use it.
The next level is pockets only. No bag at all. Some trips get there. Most don't need to. The one bag is the right answer for most travel most of the time. It is enough.