The jacket is the most capable garment most people own and the least used, because it arrived in their head attached to a suit and never detached.

Worn without its matching trousers a jacket stops being formalwear and becomes something more useful: a structured layer that adds a shoulder line to anything underneath it. The question is not whether this works. It is which jacket, and what goes under it.

What a jacket is really doing

Strip away the associations and a jacket performs three mechanical functions.

It builds a shoulder. The canvas and the sleeve head create a defined horizontal at the top of the body, and that horizontal is what the eye reads as composure. No knit, however good, does this.

It creates two vertical lines down the front. These narrow the torso visually and give the whole outfit a frame. It is the same effect a long vest produces, with more structure.

And it closes the silhouette. An outfit without a layer reads as unfinished in certain rooms, not because of any rule but because the body has no outline. A jacket supplies one.

Once a jacket is understood as doing these three things rather than signalling formality, the question of what to wear it over becomes much simpler. Anything that benefits from a shoulder, a frame and an outline.

A structured jacket in a neutral is the version that works over the widest range, and it is worth owning before any variation.

Shoulder fit is non-negotiable

Everything else about a jacket can be altered. The shoulder cannot, not without rebuilding the garment, and a jacket with a wrong shoulder is a jacket that will never be worn.

The seam should sit at the end of the shoulder bone. Beyond it, the sleeve head collapses and produces a dent below the seam. Inside it, the sleeve pulls and a diagonal crease runs from the shoulder toward the armpit.

Test it by standing square to a mirror and lifting both arms slightly forward. The jacket should move with you without the shoulder lifting off the body. Then let the arms hang and look at the back; it should be flat, with no horizontal fold below the collar.

Chest, waist and sleeve length are all adjustable and all worth adjusting. A sleeve that ends at the wrist bone, showing a centimetre of shirt cuff where a shirt is worn, costs very little to achieve and changes how the whole garment reads.

Length should cover the seat and no more. A single-breasted jacket cut to this length works over trousers and skirts alike; anything longer commits to one or the other.

Construction decides how far a jacket travels. A fully canvassed jacket holds a sharp shoulder and belongs over shirting; it looks overdressed over a T-shirt. An unstructured or half-lined jacket has a softer shoulder and reads as relaxed over almost anything, but it will not carry a formal room. If only one jacket is possible, the half-lined version covers more ground, and it is also the one that survives a hot climate.

Over a T-shirt

The most useful combination in the whole wardrobe and the one most often done badly.

The T-shirt must have some structure of its own. A soft, thin jersey collapses under the jacket and the combination reads as unmade. A heavier cotton with a defined neckline holds its shape and the pairing reads as deliberate. A cotton T-shirt with a proper rib at the neck is the right starting point.

Colour should be quiet. White, black, or a neutral close to the jacket. A printed or logo T-shirt under a structured jacket produces two competing registers and neither wins.

Keep the jacket open. Buttoned over a T-shirt the proportions compress and the look becomes stiff; open, the two vertical lines do their work and the T-shirt supplies the ease.

Fit the T-shirt close enough that the jacket does not have to accommodate bulk at the chest, and long enough that it does not ride up when the jacket is removed.

Trousers matter as much as the top. The fastest way to make a jacket look like an orphaned suit piece is to pair it with trousers in a similar but not identical colour, which reads as a failed match rather than a deliberate contrast. Either go clearly different in tone or clearly different in texture. A smooth wool jacket over a cotton trouser works precisely because nobody could mistake them for a set.

Over knitwear

The warmest and most forgiving version, and the one that carries a jacket into autumn.

Weight is the entire question. A fine knit sits under a jacket without changing the fit at all. A heavier jumper needs a jacket cut with room through the chest, and worn under a close-fitting jacket it will pull at the button and distort the lapel.

If the jacket has no room, size the knit down rather than the jacket up. A jacket that fits over a thick jumper will be wrong over everything else.

Roll necks work particularly well because they close the neckline and give the jacket collar something to sit against. Crew necks work. V-necks are the weakest option here, because the V and the lapel create two conflicting diagonals.

Avoid anything with a collar of its own under a jacket collar. Two collars at the neck is the single most common way this combination goes wrong.

Over a shirt, open

The closest this gets to traditional tailoring, and still entirely wearable without a tie.

Open the top two buttons, not one and not three. One reads as a tie that was forgotten. Three reads as a different kind of evening. Two sits exactly where the lapel wants it.

The collar needs enough structure to stand without a tie holding it. A soft unstructured collar collapses inside the jacket and the whole neckline goes soft. This is worth checking before buying a shirt intended for this use; the collar is the part that does the work, as set out in the white shirt guide.

Tucked or untucked depends on the shirt hem. A curved hem is designed to be tucked. A straight hem can go either way, provided it ends at roughly mid-fly; longer and it hangs below the jacket, which is the one outcome to avoid.

Denim deserves its own line because it is where this combination most often fails. It works, but only with an unstructured jacket and only with a dark, plain, well-fitting jean. A structured jacket over distressed or pale denim produces two garments arguing about which decade they are in. Keep the jean simple and let the jacket carry the interest.

With a kilt or skirt

A jacket over a skirt is the oldest version of this combination and the one that has aged best.

Proportion is everything. A short jacket ending at the natural waist with a longer skirt creates a high division and a long line below it, which is the most flattering arrangement available. A longer jacket with a shorter skirt inverts that and tends to read as mismatched.

Where the skirt has volume or pleating, the jacket should be clean and close. Where the skirt is narrow, the jacket can carry more structure or a wider shoulder.

Keep the hems apart. A jacket hem and a skirt hem that fall within a few centimetres of each other create a thick horizontal band across the middle of the body, and there is no colour or fabric choice that rescues it.

Most of this sits in the MINIMAL collection, and the question of how a jacket earns its place across a working week is handled in the five-day wardrobe.

Pocket discipline applies throughout. Jacket pockets are cut to sit flat and anything put in them distorts the line permanently, since the cloth stretches and does not recover. Phone, keys and wallet belong in a bag or in trouser pockets. The one exception is a pocket square, which is why that pocket exists at all.

Sleeves, cuffs and the small adjustments

The difference between a jacket that looks borrowed and one that looks owned is usually three small things.

Sleeve length. The cuff should end at the wrist bone. Over a knit or a T-shirt, with no shirt cuff to show, it can sit a touch shorter. This is the single most valuable alteration anyone can make to a jacket and the one most frequently skipped.

Rolling. Pushing the sleeves up is acceptable on an unstructured jacket and wrong on a structured one, where it crushes the sleeve head and leaves a permanent distortion. Know which one you own before you push.

Buttons. Replacing plastic buttons with horn costs very little and changes how the whole garment reads at close range. It is the cheapest upgrade available to any jacket.

And the bottom button stays undone, always, on a single-breasted jacket. The garment is cut on the assumption that it will be, and fastening it pulls the front out of line.

Shoulder on the bone, two buttons open, sleeves at the wrist. The suit was never the point.