You can spend a great deal of money on tailoring and undo most of it with the wrong underwear. Visible lines across the seat, fabric bunching at the thigh, a waistband printing through at the hip. The trousers get blamed. The trousers are usually not the problem.
This is a guide to what actually works under dress trousers, and why.
What goes wrong, specifically
Four problems, and each has a different cause.
The visible line across the seat. Caused by the leg opening of a boxer brief pressing into the thigh and printing through. The lighter the trouser fabric and the closer the cut, the more obvious it is.
Bunching and riding up. Caused by fabric with poor recovery. It stretches out through the day, stops holding position, and gathers where it should lie flat.
Bulk at the hip and waist. Caused by thick seams and heavy waistbands. Under a fitted trouser, a chunky elastic waistband is a visible ridge.
Heat. Caused by cotton, which absorbs moisture and holds it. Comfortable at nine in the morning, considerably less so by three in the afternoon.
Every solution below is really a solution to one or more of those four.
Option 1: A thong
The complete answer to the visible line problem, because there is essentially no fabric across the seat to print through. Nothing to bunch, nothing to ride up, no leg opening to press into the thigh.
This is why plenty of men who have no particular interest in lingerie wear a thong under a suit. It is a functional choice before it is anything else.
Our Forme Men's Thong is cut for this specifically. Soft microfiber that sits close without gripping and holds its shape through a full day, a contoured front that supports properly, and a narrow back that stays where you put it. Black, gray, blue and red, $62.
Best for: lightweight wool, linen, close-cut trousers, anything where a line would show.
The trade-off: no coverage across the seat, which is a preference question rather than a performance one.
Option 2: A seamless sculpted short
If you want coverage through the thigh and still want no line, this is the answer. A close-cut short with seamless construction gives you the leg coverage of a boxer brief without the leg opening that causes the printing.
Our L'Essentiel Men's Sculpted Short is built around that: smooth, lightly glossy fabric that contours through the hip and thigh, a low-rise waist that shapes without pinching, and seamless construction so there is no bulk and nothing showing through. It layers under tailoring exactly as it is meant to. Black and red, $68.
For most men in most suits, this is the highest-performing single option in this guide. It solves the line problem, the bunching problem and the bulk problem at once.
Best for: anyone who wants thigh coverage, anyone prone to chafing, and anyone who wants one piece that works under everything.
The trade-off: slightly warmer than a thong, though considerably cooler than cotton.
Option 3: A G-string
The most minimal option, and it clears the line problem for the same reason a thong does. Our Fil Men's G-String is a contour pouch, a thread of a waistband, and a fine string at the back, in high-stretch fabric that stays put. Black, gray, blue and red, $62.
Being straight about it: this is the least supportive option here. For a long day of meetings and standing, a thong or a sculpted short will serve you better. Choose a G-string when minimal is the point, not when endurance is.
Option 4: A jockstrap, with a caveat
The jockstrap's open back means nothing across the seat to print through, and the airflow is genuinely useful in a suit in summer.
The caveat is the leg straps. Under structured tailoring they are invisible. Under very lightweight summer wool or very close-cut trousers, they can occasionally register as a faint diagonal line. It depends on the trouser more than on the jockstrap.
If you want to try it, the Ligne Men's Jockstrap in matte black is the least likely to show, at $64. Check in a mirror in daylight before you commit to it for an important day. There is more on fit and wear in our guide to wearing a jockstrap.
What not to wear under dress trousers
Cotton boxer briefs. The default choice and the worst performer. Cotton holds moisture, the leg openings print through, and the fabric has no recovery so it bunches by mid-afternoon.
Loose boxers. Fabric gathers at the hip and thigh and creates visible bulk under a fitted trouser. There is nothing holding the fabric in position.
Anything with thick seams or a heavy logo waistband. Both show as ridges under close-cut fabric. Under a suit, a flat waistband is not a nicety, it is the requirement.
Anything stretched out. Underwear past its useful life has lost the recovery that keeps it in position. If the waistband no longer sits flat, it will not perform under tailoring regardless of what it cost.
The colour rule
The instinct is to wear white under light trousers. It is the wrong instinct. White shows through light fabric more clearly than any other colour, because it contrasts with skin.
Under light grey, beige, cream or white trousers, wear something close to your own skin tone. Under mid to dark trousers, black or grey is fine, and black is the safe default. This is why the men's range runs in black, gray, blue and red rather than white.
Fabric, briefly
Microfiber outperforms cotton under tailoring on every measure that matters here. It moves moisture rather than absorbing it, it has real recovery so it holds position through a full day, and it is thin enough that it does not add bulk.
Cotton is comfortable for the first hour. Microfiber is comfortable for all twelve. Under a suit, that is the only comparison that counts.
The recovery point is worth dwelling on, because it is where cheap underwear reveals itself. A piece that fits well in the shop and has stretched out by the third wear was made with elastic that had no recovery to begin with. It is invisible at the point of purchase and obvious a month later. We have written about how those decisions get made in our piece on how Luné d'Or is built.
A three-second check before you leave
Stand in front of a mirror in natural light, with your back to it, and look over your shoulder. Then sit down and stand up again and look once more. Almost every visible-line problem shows up in that second check and not the first, because sitting is what presses the fabric into the trouser.
Frequently asked questions
What underwear shows no line under dress trousers?
A thong shows no line at all, because there is almost no fabric across the seat. A seamless sculpted short is the next best option and gives you thigh coverage as well.
Why do my boxer briefs show through my suit trousers?
The leg opening presses into the thigh and prints through, especially in lightweight fabric. Cotton makes it worse because it has no recovery and bunches as the day goes on.
What colour underwear should I wear under light trousers?
Something close to your skin tone. White is the most visible colour under light fabric, not the least.
Do men wear thongs under suits?
Yes, and usually for entirely practical reasons. It is the only cut that guarantees no visible line under close-cut trousers.
Is a jockstrap a good option under a suit?
It can be. The open back solves the line problem and the airflow is useful in summer. The leg straps can occasionally show under very lightweight or very close-cut trousers, so check in a mirror first.
What is the best fabric for no-show underwear?
Microfiber. It moves moisture rather than holding it, and it has the recovery to stay in position through a full day. Cotton does neither.
The short version
If you want one piece that works under everything, buy a seamless sculpted short. If you want a guaranteed invisible line and do not mind an open seat, buy a thong. Wear something near your skin tone under light trousers. And stop wearing cotton under a suit.
Browse the full Luné d'Or men's collection. Sized S/M and L/XL, designed in Los Angeles, shipped from LA, with complimentary standard shipping on US orders and 30-day returns.
Keep reading: Thong vs G-string vs jockstrap and How to wear a jockstrap.