A skirt has to pass your widest point and then hang from somewhere narrower

Two circumferences, and only one of them holds the garment up

A skirt is suspended from the waist, or from wherever the top edge chooses to sit, and everything below that hangs off it. But before it can hang from anywhere, the garment has to get past the hip, and on most bodies the hip is considerably larger than the waist.

That is the whole design problem in one sentence. If the top edge is cut to fit the waist and the skirt is straight, it cannot go over the hip unless something opens: a zip, a wrap, a stretch fabric, or enough surplus width that the waist no longer fits. Every skirt in existence solves this in one of a small number of ways.

Where the top edge sits changes the arithmetic

A skirt worn at the natural waist has the largest gap to cross, because the natural waist is the narrowest circumference on most torsos and the hip is the widest below it. Lowering the top edge to the high hip or lower reduces that difference, which is a large part of why lower-sitting skirts feel easier to fit.

It also changes the length. Move the top edge down and the hem comes with it, so a skirt that finished at the knee in one position now sits above it. People frequently adjust a waistband upwards for comfort and then wonder why the hem no longer looks right. The band itself does real work too: a shaped, curved waistband follows the change in circumference between its top and bottom edges in a way a straight strip of cloth cannot.

Straight, A-line and everything in between

A straight or pencil cut takes the waist-to-hip difference out with darts or seams and then keeps the width constant, or nearly so, below the hip. It has to open somewhere to get on, and it needs a vent or a pleat at the hem to allow walking, because a tube the width of your hip won’t let your legs pass each other.

An A-line adds width steadily from the hip downwards. The extra width means the garment can often be got on with a smaller opening, the hem allows walking without a vent, and the whole thing is far more tolerant of a body whose measurements don’t match the chart. That tolerance is why it turns up in so many wardrobes.

Fuller cuts, gathered or gored or circular, take the principle further, and they shift the fitting problem entirely onto the waistband. Below that band the garment isn’t fitted to anything, which is a genuine freedom and also the reason a full skirt tells you very little about the body under it.

The hem is a consequence, not an independent choice

In a flared or circular skirt the hem is a much longer curve than the waist, and cutting it level requires care, because parts of the cloth are hanging on the straight grain and parts on the bias. Bias-cut sections drop over time under their own weight, which is why a new full skirt is often hung for a day or two before the hem is marked.

On a straight skirt the hem is easier to level and harder to walk in, and the vent or the pleat that solves the walking problem introduces its own fit condition. A vent that gapes open when standing still is nearly always a sign the skirt is tight across the seat, not that the vent was sewn badly. Shortening is not a neutral operation either, since a flared skirt taken up significantly loses the narrow bottom of its shape and keeps the broad part.

Riding up is a fit fault with a specific cause

A skirt that creeps upwards as you walk is being pushed there. The usual mechanism is that the hip or the seat is tight, so every stride forces the cloth to travel up to a narrower place where there is room, and the waistband follows because nothing is anchoring it.

Twisting has a related cause. A skirt whose side seams migrate towards the front over the course of a morning is usually one where the front and back are not balanced for the wearer, so the garment is finding an equilibrium the pattern did not intend. Both faults are worse over a slippery lining and better over a fabric with grip, which is why the same skirt behaves differently depending on what is underneath it.

What can be altered and what cannot

Taking a skirt in at the waist and hip is routine, since side seams and a centre back seam give a tailor plenty to work with, and shortening is among the cheapest alterations there is. Letting out depends entirely on the allowance folded inside, which on inexpensive garments is often minimal.

What cannot be fixed is a skirt that is genuinely too narrow across the seat, because the cloth needed is not there and adding a panel changes the garment. Nor can a straight skirt be persuaded to become an A-line, though the reverse is possible in a limited way.

So the order of priority when buying is the same as it is for a jacket, only pointed at a different part of the body. Fit the widest circumference the garment has to cover, then treat the waist, the length and everything else as adjustable. Almost everyone does it the other way round.