Turn a linen shirt inside out and its seams show how it was sewn. Fasten it and its buttonholes say something just as precise, and they are harder to disguise. A buttonhole is a slit cut through several layers of cloth and finished with dense stitching so that it cannot fray or stretch. In a few centimetres it brings together the quality of the thread, the care of the hand or machine that sewed it and the behaviour of the cloth itself, and on a linen shirt that last part counts for more than on most.

Linen is woven from long, smooth flax fibres, often in an open plain weave that lets air through. Those qualities make it comfortable in heat, but they also mean a cut edge in linen frays more readily than one in a tightly woven cotton, and the yarns can slide apart under strain. Linen shirt buttonholes are therefore worked harder than most: they hold a smooth, open cloth in place through years of fastening, washing and wear. The seams face a similar demand, as described in linen shirt seams.

Density comes first. A well-made machine buttonhole has its stitches set so close together that they form an almost solid ridge of thread on each side of the slit, finished at both ends with a short, firm bar. On a hastily made shirt the stitches are spaced, the cloth shows between them, and after a few washes the edge begins to fuzz. Hold the placket up to a window: through good stitching, almost no light passes. It is one of several signs gathered in how to identify premium linen.

Then the cut. Most buttonholes are stitched first and opened afterwards, and a clean, sharp cut leaves no loose fibres in the opening. Small whiskers of thread or cloth inside the slit point to a blunt blade or a rushed finish, and on linen, which naturally sheds a little fibre, a clean opening is especially telling.

In the most traditional shirtmaking, buttonholes are sewn by hand with a buttonhole stitch, each stitch looped to leave a tiny knot along the cut edge. The result is slightly raised, very strong and visibly irregular, a few stitches never quite matching their neighbours. It is slow work and one of the clearest marks of a handmade shirt, a subject explored in what it means when a shirt is truly handmade. Either method, done well, makes a durable buttonhole; done poorly, neither does.

Orientation is a quieter sign of thought. On the front placket, buttonholes usually run vertically, so the button sits in the centre of the placket and can move a little without pulling. On the collar band and the cuffs, where the strain runs sideways, they run horizontally, so the button rests at the end of the hole and holds firm. Some shirtmakers turn the lowest placket buttonhole horizontally too, for the same reason.

The hole must also suit the button. It should be just long enough for the button to pass with a gentle push; too short and it strains the stitching, too long and the shirt opens by itself. The button's material changes the measurement: a thin, hard seashell button needs less room than a thicker walnut button. Pieter Petros fastens its men's linen shirts with natural seashell buttons, with walnut on the Monte-Carlo Male; the materials are described in natural buttons: seashell, walnut and corozo.

Look, too, at how the button is attached. A button sewn tight against the placket leaves no room for the layer of cloth that must sit beneath it when the shirt is fastened, and it strains the buttonhole each time. A well-sewn button stands slightly proud on a short stem of thread, wound round beneath it, so it passes through the hole cleanly. It takes seconds to check in a fitting room.

None of this appears in a product photograph. It is found with the fingers, each morning, in the small act of fastening a shirt.

Read more: What Makes a Shirt Truly Luxury  ·  The Linen Shirt Collar Guide