A shirt collar sits on a band, and the band is what actually fits

Two pieces, and you only look at one of them

A shirt collar is not one component. It is two, sewn together: a narrow band that encircles the neck and carries the top button, and a wider shaped piece — the fall, or leaf — that folds down over the band and hides it almost completely. The number printed on the label is the circumference of the band, measured button to buttonhole, and nothing else.

Because the fall is the part you see, that is the part people blame. A collar that gapes at the front, rides up at the back or refuses to sit under a jacket is nearly always doing so because the band is wrong for the neck it is on, or because the band and the fall have stopped agreeing with each other. The fall is decoration attached to a working part.

This also explains a small everyday mystery. Two shirts marked the same neck size can feel entirely different, since manufacturers add different amounts of room to the measured neck, and a soft washed band gives in a way a heavily fused one never will.

The height of the band decides how the collar meets everything else

The band is not a uniform strip. It is deeper at the back of the neck than at the front, and that back height — the stand — governs how far the collar rises before the fall turns over. A high stand holds the collar up against the neck and shows above a jacket collar. A low one lets it settle towards the shoulders.

That relationship matters more than it sounds. A tailored jacket is cut on the assumption that roughly a centimetre or so of shirt collar will be visible above it at the back, and the shirt collar in turn is meant to sit against the neck rather than float away from it. Get the stand too low and the jacket collar rubs directly on skin. Too high and the shirt looks like it is swallowing you.

The stand also creates the space a tie knot occupies. A collar with a shallow stand has nowhere to put a substantial knot, so the knot pushes the collar points outwards and the whole front distorts.

Spread is geometry, and the rules around it are softer than people claim

Spread is the angle between the two collar points, or more usefully the distance between their tips. A narrow point collar keeps the points close together and close to the neck. A wide cutaway swings them out towards the shoulders, leaving a broad gap in the middle that a tie knot is expected to fill.

There is a real mechanism underneath the advice you usually hear. A wide spread puts horizontal lines near the top of the chest and a narrow one puts near-vertical ones, and those lines sit right beside the face, which is where people look. Beyond that, most of what gets said about which face suits which spread is convention rather than optics, and it is worth holding loosely.

What is not a matter of taste is the pairing with a knot. A small knot inside a very wide collar leaves the shirt showing through the gap, and a bulky knot inside a narrow collar forces the points apart. Match the volume of one to the space left by the other and the arrangement looks deliberate.

A collar that will not lie flat has usually failed internally

Both parts of a collar contain a hidden stiffener, either fused on with adhesive or stitched in as a separate layer. When that stiffener is bonded and the bond breaks down — from repeated hot washing, from steam, from age — the outer cloth separates from it in patches and the collar develops a rippled, bubbled surface that pressing cannot remove.

A second, quieter failure comes from differential shrinkage. The band, the fall and the interlining are three different pieces of cloth, and if they shrink at different rates the collar begins to curl, because one layer is now shorter than the layer it is stitched to. You see this most on shirts that have spent years going through hot cycles.

Removable stiffeners in the point tips are a separate matter altogether. They are simply thin strips that hold the points straight, and losing them makes the points curl while leaving the rest of the collar intact.

What can be changed, and what is fixed the moment you buy it

Very little of a collar is adjustable. Moving the top button gains perhaps a centimetre of neck room, which is genuinely useful and costs almost nothing, but it slightly shifts where the two halves of the band overlap and can leave the collar sitting a fraction off centre.

Letting a band out properly means opening it and using whatever seam allowance is inside, and there is rarely much. Replacing a collar entirely is a real operation that good shirtmakers will do — sometimes turning the old one, more often fitting a new one — and it makes sense on a shirt whose body is still sound.

The shape of the fall, though, cannot be altered at all. Spread, point length and stand are cut into the pattern, so if you dislike how a collar looks the only remedy is a different shirt. Which is an argument for deciding about collars before the till rather than after.