Hold a linen shirt up to a window and the weave shows itself. Among the regular lines of warp and weft there are places, a few millimetres long, where a thread thickens and then returns to its ordinary width. These are linen slubs. People meeting them for the first time sometimes take them for faults and wonder whether the garment should go back. It should not. Slubs are part of what linen is, and, read properly, they are one of the more informative things about a piece of cloth.

They begin in the plant. Flax fibre does not come as a single even strand. It lies in the stem as bundles of very fine cells, of differing lengths, bound together by natural gums, and the work of turning a stalk into yarn is largely the work of separating those bundles and laying them parallel. The separation is never perfect. Where a few fibres remain bound together, or overlap as they are drawn out and twisted, the yarn is briefly thicker. Woven into cloth, that thicker place is a slub.

How many slubs a cloth has, and how large, depends on two things: the fibre and the spinning. When flax is combed, the long fibres are separated from the short. The long ones, called line, spin into a fine, smooth yarn. The short ones, called tow, give a hairier and more uneven yarn with more slubs in it. Spinning adds its own effect. Fine shirting yarns are generally wet spun, passed through warm water that softens the gums so the fibres can be drawn out thin and even; dry-spun yarns are fuller and more rustic. The distinction begins with long-staple linen fibre.

This is what makes slubs a guide to quality, provided they are read with some care. In a fine shirting linen they are present but small, sparse and scattered without pattern, in a cloth that is otherwise even. A fabric crowded with heavy slubs may be a deliberate rustic choice, right for a heavy overshirt or a furnishing cloth, or it may simply have been spun from shorter fibre. The label and the hand together usually make clear which. A slub is not a mark of luxury in itself, and more of them does not mean more authenticity.

Two cases deserve suspicion. The first is a cloth sold as linen in which the slubs recur at intervals, the same size and the same spacing, like a printed motif. Nature does not repeat itself so neatly, and such a pattern suggests a yarn engineered to look irregular. The second is the opposite: a so-called linen with no variation of any kind, glass-smooth from selvedge to selvedge. It may be very fine indeed, or it may not be all linen, and the composition label is worth a second look.

A slub is easily told from a true fault. A fault is a break in the structure: a hole, a thread that has snapped or is missing so that the cloth is thin in a line, a hard knot standing proud of the surface, a stain woven in. A slub is the reverse, a place where there is slightly more yarn and not less, lying flat within the weave and continuous at both ends. Drawing a fingernail lightly across it settles the question. A slub feels like a gentle swelling in the thread; a fault catches.

Slubs do not, as a rule, shorten the life of a shirt. A garment in good linen can wear for many years with every one of them intact. What they do affect is appearance, and for the better. The thicker places take dye a little differently from the yarn on either side, so a single colour is never quite flat: there is a fine fleck in it, more visible in deeper colourways such as Dark Blue or Dubarry than in White or Oyster. It is the reason plain linen looks alive where a perfectly uniform cloth looks blank.

They need no special care, only a little restraint. Do not pick at a slub or try to pull it out, since it is part of a thread that runs the width or length of the garment. Do not shave it. Wash linen at 30°C on a gentle cycle, dry it in the air, and iron it while slightly damp if a smooth finish is wanted. Over the first several washes the cloth softens, the slubs settle into the weave and become less distinct to the touch, though they remain visible against the light.

The garments in the PP collection, from the men's linen shirts to the Amelia linen dress shirt and the PP linen blouse, are linen, much of it certified Belgian linen, and they show what linen shows: a clean, even cloth with the occasional small slub in it. A buyer who knows what that mark is has one more way of judging what is in the hand, alongside weight, weave and the other signs described in our guide to identifying premium linen.

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