My sister called me from a dressing room at Target, which is a sentence that should tell you everything about how this conversation was going to go. “Do you wear underwear under this?” she asked, no hello, no context, like we’d been mid conversation for twenty minutes already. I said under what. She said the swimsuit. I said which one. She said it doesn’t matter, Morgan, answer the actual question.
And I couldn’t. Twenty some years of getting dressed, styling other people’s bodies for a living, and I had never once consciously decided the answer to that question. I’d just always… not. It wasn’t a rule I followed. It was a rule I didn’t know I had.
That bothered me more than it should have, honestly. I spent the rest of that day thinking about it instead of doing the invoice I was supposed to be doing.
Why the answer is no, and why almost nobody actually says why
Here’s the thing nobody explains when they tell you not to. It’s not modesty. It’s not a hygiene rule handed down from someone’s grandmother, although I’m sure that version exists in someone’s family too. It’s fabric behavior, plain and boring and completely explainable if a single person ever bothered to.
Swimsuit fabric is built to do a job your regular underwear was never designed for. It’s cut to sit against skin and stretch four directions at once and dry out without falling apart, and most everyday underwear, cotton especially, does none of that. Cotton holds water. It gets heavy, it sags, and then it does the thing that ruins the whole look, which is show through the swimsuit fabric in a wet outline that’s somehow both see through and shapeless at the same time. I had a client a couple years back, someone I was styling for a resort trip, who brought her regular bikini briefs to layer under a one piece “for coverage,” and we caught it in a fitting room mirror before she ever got near a pool. Coverage was the opposite of what happened. It was the least covered that suit ever looked.
There’s also the plain mechanical issue of two elastic systems fighting each other. A swimsuit has its own leg opening, its own grip, engineered by someone who thought about where it needs to sit on a body that’s about to be horizontal in water. Underwear has a completely different elastic doing a completely different job. Put them together and you get bunching, riding, a line digging in where no line was supposed to be, and none of it visible until you’re standing up out of a lounge chair in front of people.
I measured this once out of pure stubbornness, because that’s apparently who I am now. Took a suit labeled as full coverage, laid a pair of regular briefs over the liner, and the underwear’s waistband sat almost an inch inside where the suit’s own leg line hit. An inch. That’s not a small gap, that’s an entire separate garment trying to assert itself through fabric that was never made to hide two waistbands at once.
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Where it actually gets complicated, because it does
I want to be fair here, because I’ve spent this whole piece sounding very sure of myself, and the honest answer is it’s not always this clean.
Built in liners are doing less work than the marketing suggests. I’ve measured suits labeled with “full support lining” that turned out to be a single layer of the same stretch fabric as the outside, just doubled, with zero actual structure to it. Labeled as supportive, functioned as decorative. That’s the flattering conversation again, a word that gets slapped on a garment because it sounds nice, not because anyone checked what it’s doing.
So no, I’m not telling you to trust every liner blindly. What I’m telling you is the fix isn’t cotton briefs, it’s a proper swim specific layer if the suit’s own lining genuinely isn’t cutting it.
There are seamless swim shorts and swim specific bottoms made from the same quick dry material as the suit itself, built with that in mind rather than borrowed from a different job entirely. That’s a completely different decision than reaching for whatever’s in your regular underwear drawer because it felt like the safer instinct.
And for a period specific situation, that’s its own separate conversation, a period swimsuit or a suit specific product built for exactly that, not a regular pad and definitely not regular underwear layered underneath hoping for the best.
My sister ended up buying the suit with nothing under it, texted me a photo from the pool two weekends later looking completely unbothered, like the whole dressing room phone call had never happened. And that’s sort of where I landed too. Not as some hard universal rule I’m handing down, but as the boring, unglamorous, actually correct answer. The suit was built to be worn alone. My job, and I’ll admit it took an embarrassingly overthought Tuesday to get here, was just to trust that the engineering already did what I kept trying to double up on myself.
