Like an airline’s carry-on rules, the laws of traveling in the city are oriented toward the compact and lightweight. The holdall (a.k.a. duffel or weekender) should fit in an overhead bin — or a hotel’s bag holder. One personal item should fit your surroundings — a proper tasteful backpack that looks at home in an urban environment, not packed for an overland expedition. And beyond one or two pieces, that’s all you need. Don’t overdo it, or the city (like an airline) will make you pay — in discomfort, if not cash.