The parts of a bra, and the parts of a knicker

A cup letter is not a volume. It is the difference between your bust and your ribcage, in inches, one letter per inch.

That is why the letter means nothing on its own. A C is 3 in of difference, and a 36C holds a full cup more than a 34C, because the same difference measured on a bigger ribcage is a bigger cup.

So the letter always travels with the band. Change the band and you must change the letter the other way to hold the same amount. That is what a sister size is.

Above D the ladders stop agreeing. A DDD in one brand is an E in another and an F in a third, and nobody is wrong, they are just using different alphabets. We print your size in each brand's own notation rather than expecting you to convert.

Cut and sew, 3-part, 4-part

A seamed cup, built from pieces. The most supportive kind, and support rises with the seam count.

Moulded, seamless

One piece, shaped by a machine. Smooth under clothes, and it holds one shape whatever your shape is.

Contour

A thin foam layer, for modesty rather than lift. Often seamless.

Demi, balconette, half cup

Less upper coverage, straps set wider. Good under a wide neckline, harder to keep on a shallow upper breast.

Minimiser

Shallower and wider than average, so projection is redistributed rather than removed.

Support sling

A hammock of firm fabric across the bottom and sides of the cup.

Support panel

A vertical strip along the underarm edge, pushing side tissue forward. Worth having from a C upward.

The direction of a seam is the shape it makes. Read the seams and you know what the bra will do to you before you try it.

A seam is where the fabric changes direction, so it is where the cup can hold a shape instead of taking yours. That is why a seamed cup supports better than a moulded one at the same price.

It is also why a moulded cup gaps. A machine formed it into an even shell, and most breasts are not even, so the volume sits low and the top of the shell has nothing in it.

The wire sets the width of the cup, not the volume. Its job is to sit on bone, all the way round, and it is doing that or it is hurting you.

A wire has a diameter and a length, and both are chosen by the maker for that size and that style. The letter you buy picks the projection. The band you buy picks the width.

Sister sizes share a wire. The wire in a 36B is the same physical piece as the one in a 34C and a 32D, which is exactly why those three hold the same amount.

Wires are sprung. They splay open when you fasten the bra and close again when you take it off, and that outward tension is part of what supports you. A wire that snaps mid-span was carrying a load it was never cut for, which means the cup was too small, the band was too small, or both.

Length is a style decision. A plunge and a demi use a short wire, a full cup and a strapless use the longest, because a strapless wire is doing the work the straps normally do.

The band carries the weight. Everything else in the bra is holding a shape; this is the only part doing the lifting.

It should be snug on the loosest hook of a new bra, so that the tighter hooks are left for later, when the elastic has given. A band bought on the tightest hook has nowhere to go and is finished in months.

What it is made of decides how long that takes. Powernet, the firm knit also used in compression garments, resists stretch and holds its shape. A band that stretches easily end to end is worn out before the cups look tired.

A bandless or floating-cup bra has no band under the front at all, only fabric joined to the outer cup edges. It looks lighter and it supports less, and the bigger the cup the more that matters.

Also called the bridge or the gore. Judge it against your apex line: the horizontal through your nipples. A bra that supports you has the panel finishing at or above that line.

The panel holds the two cups apart and stops them travelling. It should lie flat against your breastbone. If it lifts away, the wires are not sitting where they should and the cups are almost always too small.

Width is a shape question, not a size one. A wider panel suits wide-set or splayed breasts, a narrow one or none at all suits close-set. Panel width narrows as cups get bigger, because bigger breasts sit closer together, and panel height rises for the same reason.

A plunge has a strip of tape there, or nothing. That is a real trade: with a large cup and softer tissue you can fall out of it when you lean forward, and no size fixes that because the support was designed out.

The wings are the sides and back, usually one piece from the cup to the fastening. They are what decides whether you get a bulge.

Do the fabric test in the shop. Take the wing in both hands and pull. If it stretches easily and stays stretched, it will be exhausted in a few months and it is not supporting you now.

Coverage and line matter more than tightness. More back coverage and a curved edge spread the pressure. A narrow back on a firm band presses into one line, and back tissue softens with age, so anyone can get that line eventually.

Rotate three bras so none is worn on consecutive days. Elastane needs about a day to recover its shape, and giving it that day is the cheapest thing you can do to make a bra last.

A new bra should be snug on the outermost set of eyes. The tighter sets are the life left in it.

Hook count is not support. Three rows on a soft stretchy band support less than two on powernet, and makers choose the count to suit the back they have designed.

When you can only wear a bra on the tightest hook, that bra is done. Band elastic does not recover, and nothing you do to the straps replaces it.

Straps are stabilisers, not load-bearers. They should carry no more than about a tenth of the weight, and if yours are digging, they are carrying far more than that.

So a strap that digs is a band problem. Tightening it moves more load onto your shoulders, which is why it feels better for an hour and worse by the evening.

A strap that slips is usually mounted too far out: set wide at the front, wide at the back, or sewn on at an angle. A ring sewn flat onto the top of the cup lets the strap swivel, which is the worst combination if you have sloping shoulders.

A shallow upper breast slips straps too, because the hollow at the top of the cup leaves the strap nothing to sit against. That is a cup shape to solve, not a strap length.

The front attachment tells you what it will do. Directly above the apex gives the most support. Set out toward the shoulder, as on a balconette, opens the neckline and pushes tissue toward the centre.

The hardware that adjusts strap length sits at the back for three plain reasons, and none of them is fit.

It keeps the front strap clear for lace or trim, it keeps metal off the top of your shoulder where it can sit under a bag, and it keeps a lump from printing through a close-fitting top.

Check the rings before you buy. A ring stitched flat to the cup or to the back lets the strap rotate, and a rotating strap wanders off a sloping shoulder no matter how you set the length.

Rise is the distance from the gusset to the top of the waistband. It is the single measurement that decides whether a knicker stays where you put it.

Your body has a narrowest point at the waist and a widest at the hip. A waistband that finishes above the waist is sitting in a groove and has nothing to climb toward. One that finishes below it is sitting on a slope, and it will travel up to the narrow point and fold when it gets there.

So rolling is a rise problem, not a size problem. Going up a size gives you a looser band on the same slope, which rolls sooner.

Width matters after height. A wide flat band spreads the pressure and lies down. A narrow rolled or elasticated band is a rope, and a rope prints through everything.

The panel across the front is where lines come from, and it is decided by two things a label rarely prints: where the seams sit, and how heavy the fabric is.

A seam is a ridge. Under a knit skirt or a light trouser, the ridge is the line you can see, and no size change moves it, because the seam is sewn where it is sewn.

Fabric weight does the rest. A thin fabric under a thin cloth shows the edge of itself. Going up a size makes a looser edge, not an invisible one.

Bonded or laser-cut edges have no seam and no hem at all. That is the only construction that reliably disappears, and it is worth paying for if lines are your complaint.

The panel at the base. Its width has to match the difference between your hip and your waist, because that difference is what pulls the cut in.

A wide gusset gives the whole garment somewhere to sit. A narrow one gives it nothing, so the fabric follows the pull and goes where you do not want it.

This is why the same style rides in on one woman and not another at the same size. The bigger your hip-to-waist difference, the more the cut is being pulled, and the wider the gusset has to be to resist it.

Cotton, always, whatever the rest of the garment is made of. This is health, not fit, and a synthetic gusset is worth avoiding at any price.

The finish around each leg opening. Narrow elastic on a soft thigh is a wire on a breast: it concentrates the pressure into one line instead of spreading it.

Chafing comes from that line moving. A narrow band grips in one place, your thigh moves, and it saws.

Wide, flat, and bonded solves it, because a wide band spreads the same grip across more skin and has no hem to abrade. Picot edging, the decorative scalloped elastic, is the worst of both: narrow and textured.

A looser leg does not fix chafing. It moves more, which is the cause.

The joins at your hips take the whole sideways load. How wide they are decides whether you get a bulge above them.

A wide side panel spreads the pull across your hip. A string side or a narrow join concentrates it, and soft tissue moves around a concentrated line, which is the bulge.

This is the same mechanism as a narrow bra wing. Nothing about it is a fault in your body, and going up a size loses you the hold without moving the line.