Packing light is usually taught as subtraction, which is why it fails. Take fewer things and you spend the trip rationing them. The version that works is substitution. Choose items that each do two jobs, and the case gets smaller without the wardrobe getting thinner.

A light knit is the clearest example. It covers shoulders in a cold restaurant, it goes over a swimsuit on the walk back, and it works alone as a top in the evening. That is three functions in one item, which is why it earns space that a second dress does not.

The eight here run $160 to $495. Negative's waffle knit pullover at $160 and Anemos' knit tank at $165 are the lightest. Posse's Aimee at $200 and Zulu & Zephyr's crochet tank at $210 are the open-weave versions for real heat. Third Form's Sonnet turtleneck is $220 and FRAME's rib-knit polo is $298. Ramy Brook's feather-trim crewneck at $395 and gwyn's classic V-neck cardigan at $495 are the two that also work as the evening layer.

Plain tops are the load-bearing part of a light case and the part people underbuy. Four plain tops and two bottoms produce more combinations than four printed tops, because plain pieces stack with each other and prints do not.

The quality test is the same as always and it is not the label. Hold the fabric up to light. If the weave shows through clearly it will go transparent when damp and shapeless after two washes. Pima cotton, heavier jersey and slub weaves survive travel. Thin viscose does not.

The eight here run $95 to $320. Éterne's 90's jersey tank at $95 and rag & bone's ribbed tank at $98 are the base layers. MAJESTIC FILATURES' soft touch tee at $125 and Eileen Fisher's crinkled halter at $138 sit in the middle. ALICE + OLIVIA's cowl-neck blouse is $195 and Veronica Beard's puff sleeve is $198. gwyn's Marney T-shirt at $295 and posse's Cody top at $320 are the expensive plain pieces, which is a real category and not a contradiction.

Shorts are the most space-efficient garment there is and the one most likely to be left at home out of nerves. The compromise that solves it is length and cut rather than avoidance. A boxer or a relaxed short in cotton reads as clothing rather than sportswear, and sits closer to a skirt than to a running short.

Fabric decides whether they work in heat. Cotton poplin, seersucker and linen all hold a shape away from the leg. Anything with stretch clings once the day warms up.

All eight here run $95 to $195 and every one is cotton or a cotton blend. With Nothing Underneath's seersucker boxer at $95 is the cheapest and the coolest. Comme Si's poplin boxer is $140 and Les Tien's Serena cotton short is $145. ELCE's ribbed knit at $150 and LA VESTE's striped mini at $160 are the more fitted pair. Agolde's Ramona denim at $188 is the one that works in a city. Chan Luu's dupioni at $190 and Éterne's cotton track short at $195 top the range.