Briefs that do not show under tight trousers

Short answer: Short answer: Choose laser cut edges or a style with a wide, flat elastic. It is the edge, not the size, that shows through the fabric, no matter how well the pants otherwise fit.

The edge is the whole problem

A narrow elastic creates a line across, because it presses a thin groove into the skin that the fabric above cannot hide. A wide, flat edge spreads the pressure over a larger area and therefore does not show, even when you sit down. See bamboo panties.

Color choice against light pants

Under light pants, skin tone is always the least visible, because it sits close to the skin's own tone and creates no contrast. White creates more contrast against the skin, because the two white shades are rarely exactly the same. See bamboo underwear for women.

Not a size smaller

A style that is too small cuts in and creates more lines, not fewer, because it pushes the skin up around the edge instead of lying smooth. Choose the right size with a better edge instead of a smaller style that digs into the skin all day.

The style matters

A thong is not the only solution, even though many think so. An ordinary style with a flat edge is just as invisible and more comfortable for most people, because it spreads the fabric over a larger area of the body.

Fabric thickness matters

A thin bamboo fabric sits closer to the body and creates less volume under the pants than a thicker fabric does. Bamboo viscose is soft enough to move with the body instead of holding its own shape under tight fabric.

The pants fabric decides visibility

Thin, tight pants, say in jersey, reveal more than thicker denim, because less fabric means less to conceal what lies underneath. If the pants fabric is very thin and tight, choose the most seamless style you can find.

Seams show in more places than the edge

Even a good elastic can be undermined by a visible seam at the crotch or hip, where fabric is sewn together. Look for styles with flat seams all the way around, not only at the top edge, if you want to avoid every visible line.

How to test before you go out

Stand in front of a mirror in the pants you plan to wear, and turn to the side and back in ordinary light. Sit down for a moment too, because edges that are invisible standing often appear once the fabric stretches over a seat.

Washing keeps the elastic flat

High heat in the dryer can make the elastic stiffer and more likely to show through thin pants over time. Wash at a low temperature, so the edge stays soft and flat for as long as possible, and the underwear keeps sitting invisible like new. The same rule about smooth fabric applies to bamboo tops for women under a thin top.