The undershirt that takes the sweat so your shirt doesn't

Quick answer: Buy a thin, snug, short-sleeve undershirt in a shade close to your skin, not white. It should be long enough to tuck in. It absorbs the sweat, and your good shirt never has to.

Why your white shirt turns yellow

Open the closet and look at the armpits of your favorite white shirt. Yellow, stiff, a little crusty. You've washed it a dozen times and it's still there.

That yellow isn't only sweat. It's sweat reacting with your antiperspirant and baking into the fibers of a shirt that gets worn, washed and dried again and again. Once it sets, scrubbing mostly makes you angry.

So stop fighting it at the laundry stage. Put something between you and the shirt that's built to take the hit and cheap enough to retire. That's the whole job of an undershirt. Staying invisible is the second job, and it's where most men get it wrong.

Skip the white one

This is the contrarian part. The white undershirt feels like the default, and it's the one that shows.

Under a white or light blue shirt, a bright white tee reads as a hard bright line across your chest and shoulders. Your skin isn't white. A shade close to your own skin tone blends into the shirt and disappears. Lighter skin, go for a light tan or beige. Darker skin, go for a true brown, closer to the real thing than you'd guess. Hold it up against your forearm. If it vanishes there, it'll vanish under a shirt.

Gray works under darker shirts and in a pinch. Black under a white shirt is a mistake, and you already know it.

White is fine under a dark shirt that's thick enough, or for a man who never wears light colors. Most men wear light shirts to work.

Sleeves matter more than you'd think

The stain zone is the underarm. A tank-style undershirt, the A-shirt, cuts away right where you need coverage, so the sweat goes straight to the dress shirt anyway.

Get a short sleeve or a cap sleeve. The sleeve seam should sit in the underarm and cover the whole area. It also has to stop well short of the end of your dress shirt sleeve. If it peeks out below a short-sleeve shirt, it's too long. Check this in the mirror with your arm raised, not hanging at your side.

I'd skip the ones with sewn-in underarm shields. They tend to print as a patch through a thin shirt and they hold heat right where you're already sweating.

Neckline: pick by what's on top

• Open collar, no tie: a deep V. If you see white at your throat, it's too shallow.
• Buttoned collar with a tie: a crew or a modest V. Either works, and the crew also covers the back of your neck, where collars yellow too.
• Polo or henley: a deep V again, or go without and wear a polo that doesn't cling.

If you're not sure what your shirt wants, the guide on what smart casual actually means covers when collars open and when they don't.

Fit decides whether it shows

An undershirt that's too loose bunches at the waist and you'll see ripples through the shirt. One that's too tight prints a ridge at the edge of the sleeve and shows every seam.

You want it close, like a second skin, without squeezing. You should be able to pinch about an inch of fabric at the side of your ribs. Any more and it bunches.

Length is the other half. It should be long enough to tuck three or four inches into your trousers. Raise your arms. If it pulls out, it's too short. Tuck the undershirt first, then the dress shirt over it, so the two don't ride up independently and bunch at your belt.

For the full routine on checking a garment in the mirror, use the checks that tell you a garment actually fits. They apply here exactly as they do to a jacket.

Fabric, in order of what I'd reach for

Thin cotton jersey. Plain, light and absorbent. It does the job and costs the least. Pick a lightweight one. A heavy tee under a dress shirt is a sweater in July.

Cotton and modal blends. Modal is a soft fiber made from beech trees. Blended with cotton, it feels lighter on the skin and drapes smoother, so it prints less under a fitted shirt. A good choice if you wear slim shirts.

Merino. It costs more. It's also less clammy than you'd expect, and it's the one to try if you hate how a wet cotton tee feels by mid-afternoon.

Skip: shiny gym polyester. It clings, it prints, and it holds on to odor longer than cotton does. Great for the gym. Wrong under a shirt you wear to meet people.

The trade-off nobody mentions

You're adding a layer. In real heat, that's one more thing between you and any breeze. If you work outside in August, an undershirt under a long-sleeve dress shirt can feel worse before it feels better.

The answer isn't to quit. It's to go thinner, not thicker, and to give it a fair trial. A good thin undershirt, in the right fabric, often feels about the same, and a dry shirt is more comfortable than a soaked one. But if you're in a hundred degrees with no shade, there's no shame in a short-sleeve shirt and one shirt only. Just accept that one will get washed more often.

Make it last, and make the shirts last

• Wash it after every wear. It's the layer that does the work, so it needs washing.
• Let your antiperspirant dry for a couple of minutes before you pull anything on. Wet product on fabric is how the crust starts.
• Pretreat the underarms of old stained shirts with a little dish soap or an oxygen-based stain remover before the wash. It won't always win, but it beats scrubbing.
• Keep it out of the dryer. Heat sets stains and wears fabric out early, and it shrinks the undershirt until it rides up. The dryer is why your clothes wear out early covers it. Hang them. A thin undershirt dries in an hour.
• Own five or six. Cheap ones are fine. Rotate them and replace them when they go gray or thin, not when they tear.

What to buy this week

One pack of three, in a skin-tone shade, short sleeve, V-neck. Wear them for two weeks, including a day when you're running around in the heat. By the end, the fit, the neckline and the fabric will tell you what to buy more of.

If you're building a work week, an undershirt belongs on the list next to the garments in Ten garments that get you through almost any week. It's the one nobody sees, and it's why the rest still look good in year three.