Flannel ★

Finish, not structure

Two cloths can be woven identically and only one is a flannel, because flannel is defined by what happens after the loom. Milling shrinks and felts the cloth; raising lifts fibre ends to the surface; cropping levels them. The weave underneath disappears into a soft fuzz.

Woollen-spun flannel (carded, lofty yarn) is softer, warmer and fuzzier. Worsted flannel is spun from combed yarn and then milled, so it keeps more definition and wears harder. Both are flannel; they behave differently.

Why grey flannel trousers

Because the nap absorbs light and hides everything. A grey flannel trouser is forgiving of creases, of sitting down, of a slightly imperfect fit, and it goes with essentially every jacket in a wardrobe. It is the most useful single item in a classic wardrobe and the most frequently recommended first purchase for anyone building one.

The English tradition (Fox Brothers in Somerset is the reference) associates flannel with weight, and a 13- or 14-ounce flannel trouser hangs in a way a light one cannot. Light flannels exist and are comfortable; they bag at the knee sooner.