The third piece is the layer that turns two garments into an outfit. A top and a bottom is clothing. Add a cardigan, a scarf or a belt and it reads as deliberate. This is the oldest styling idea there is and it holds because it costs almost nothing in luggage space while changing how everything else looks.

On a warm trip the third piece has a second job, which is temperature. Evenings drop, restaurants over-cool, and boats are windy in a way that nobody plans for.

The eight knits here run $195 to $325 and are all cotton, linen or a blend of the two rather than wool. Onia's linen cable-knit top is $195 and Posse's Nadine striped cotton is $220. DL1961's July linen-cotton tank is $229. Third Form's Bound linen-blend cardigan at $260 is the only true cardigan in the grid and the most useful single item in it. Les Tien's zip-up hoodie is $275, Baobab's Corsica cutout top is $295, Éterne's cropped half-zip is $295, and THE GARMENT's striped knit polo is $325.

A scarf is the lightest third piece available and the one that changes an outfit most per gram. Worn at the neck it fills a plain neckline. Tied to a bag handle it changes a bag. Over the shoulders it covers a sunburn, and around the head it solves a hair problem. Nothing else in a case does four jobs.

Silk is the right fibre for this particular use because it packs to nothing and does not crease permanently, which cotton does.

The eight here run $115 to $625. Bembien's Simone silk scarf at $115 is the entry point and the easiest to lose without regret. PARDOhats' Sahara beaded leopard print is $295 and Otzio's Bosco silk twill is $305. Prada's printed silk twill at $330 and Toteme's Hankie lace silk at $340 are the two most versatile squares. Max Mara's silk scarf is $475. Dolce and Gabbana's floral twill at $545 and Burberry's EKD crest at $625 are the collector pieces at the top of the range.

A belt is the third piece that costs nothing in space and is the one most often left behind. It defines a waist on a loose dress, it holds an oversized shirt in place, and it makes a plain trouser and tee look considered. It also survives every trip, which is not true of most things in a case.

The variable worth thinking about is the buckle rather than the leather. A heavy buckle imprints through a light fabric when you sit down. A slim one does not.

All eight here run $200 to $350 and all are leather, suede or a mix. Black Suede Studio's Sable at $200 is the plainest. Chan Luu's sequined tie belt at $245 is the only soft-tie option and works over a dress without a buckle line. Dries Van Noten's snakeskin-effect is $275 and TWP's narrow leather is $325. Le Sundial's tassel belt is $330, and All Three's Ezara suede, Déhanche's alligator-embossed Hollyhock and CHIS STUDIO's Saint Tropez all sit at $350.