Flax is pulled, not cut. At harvest the plant is drawn out of the ground by the root, because a blade would shorten it by the few centimetres left in the stubble, and those centimetres matter. That one fact explains most of what distinguishes fine linen from ordinary linen. The phrase long-staple linen fibre is borrowed from the language of cotton, and it points at the right thing: the length of the fibre, more than any finish applied afterward, decides how a linen shirt feels, how it wears and how long it lasts.

The plant stands about a metre tall when it is ready, roughly a hundred days after sowing. Its fibre runs the length of the stem, in bundles lying just beneath the surface, from root to flower. Very few fibres in a wardrobe begin at anything like that length. After pulling, the stems are laid in rows on the field to ret: dew, rain and sun, over several weeks, loosen the bond between the fibre and the woody core. The grower turns the rows and waits. It is slow, it depends on weather, and it cannot be hurried.

The retted straw then goes to be scutched, which breaks away the core and frees the fibre, and hackled, which combs it through finer and finer pins. Here the harvest divides. What remains long, parallel and lustrous is called line: strands that can exceed half a metre. What breaks off short is called tow. The two are spun and sold separately, and they make different cloth. The whole of the journey is followed in how linen is made.

Length changes the yarn. A long fibre lies alongside its neighbours for much of its length, so it can be spun fine and smooth with relatively little twist. A short fibre has to be twisted harder to hold together, and every one of its ends is a point that can lift from the surface. Yarn spun from line is even, strong and faintly glossy. Yarn spun from tow is thicker, more irregular and hairier. Both are linen. They are not the same linen.

In a shirt, the difference is felt before it is seen. Cloth woven from long fibre is smooth and cool in the hand, with a quiet lustre in the light. It can be woven closely without becoming stiff, so it is fine and opaque together. It sheds little lint and resists the small surface balls that short-fibred cloth can form with friction. Above all it ages well. Each wash relaxes the yarn a little further, and the shirt grows softer for years without growing thinner. Cloth from short fibre often feels pleasant when new, and then loses its body.

Where the flax is grown has a great deal to do with it. The coastal belt of western Europe, from Normandy through Flanders to the Netherlands, has the conditions the plant prefers: deep soil, a mild maritime climate, regular rain, and generations of growers who know when to pull and when to turn. The larger part of the world's long flax fibre comes from this strip of land. It grows on rainfall, and in rotation with other crops.

This is the background to the Belgian Linen mark. It is a certification mark, not a description of style, and it attests that a cloth was woven in Belgium from flax of European origin. For a buyer it is useful because it is specific: it names a place and a standard that can be verified. PP works in certified Belgian linen for the core of its collection, and certified garments are accompanied by a certificate of origin. The subject is treated in full in the guide to 100% Belgian linen.

A few signs can be read without a laboratory. Look across the surface of the cloth in raking light: fine linen is even, with the small natural slubs of flax but no coarse knots or thin patches. Feel it: it should be cool, smooth and dry, with some body. Look for loose fibre standing off the surface; there should be very little. Ask where the cloth was woven and whether that can be shown. And consider the price of the garment against what it would cost to do all of this properly. Long fibre is the smaller share of every harvest.

The rest is making. A shirt in fine linen deserves buttons of seashell or walnut and a hand that cuts and sews it with patience; the garments of the men's linen collection and the women's linen collection are made that way, in the Dubai atelier and in Belgium. But no atelier can add length to a fibre. That is decided earlier, in a flax field on the coast of western Europe, on the morning someone chose to pull the flax by the root.

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