The layer under everything decides how your clothes feel

By 2 p.m. the shirt has come untucked on one side, the sock has crawled under your heel, and you've adjusted something three times in a meeting. None of that's your dress shirt's fault. It's the layer nobody sees, and it's usually a fit problem, not a quality problem.

Three numbers fix most of it: a 6-inch inseam instead of a 5, a "tall" undershirt instead of a regular, and a sock sized 10-13 instead of 9-11 when you wear a 12. That's it. You can spend a hundred dollars on merino and still get it wrong if the sizing is off.

Undershirts: check the armhole and the hem, not the label

Hold a crew-neck undershirt up against a dress shirt you already own. Line up the shoulder seams. If the undershirt's sleeve ends past your mid-bicep, it'll bunch, and that bunch prints through a poplin shirt like a ring.

Then check length. Most standard undershirts are cut for a torso shorter than the average adult man's, which is why they creep out of the waistband by lunch. A tall version adds roughly two inches of body length and solves it permanently. If the brand doesn't offer tall, that brand doesn't want your business.

The neckline matters less than people think, with one exception. A deep V that shows a triangle of chest under an open collar reads like 2012. Go crew under a tie, and a shallow V under an open dress shirt, one where the collar covers the undershirt completely when the top button's undone.

Now fabric. Pure cotton feels great and stretches out at the neck and belly after a summer. A 95 percent cotton, 5 percent elastane knit holds its shape through far more washes. Modal and micromodal blends run thinner and slicker, which is what you want under a fitted shirt, though they pill faster under a backpack strap.

One color note that saves people real embarrassment: a white undershirt under a white dress shirt shows as a bright rectangle. Heather gray or a light tan disappears. Under navy or black, gray or black.

And a concession. Under a polo in July, an undershirt is just a second layer of heat. Skip it. The undershirt earns its place under dress shirts, sweaters and anything you'd rather not wash after every wear.

Underwear: inseam length and a waistband that hasn't died

Two things do all the work here. Leg length and elastic.

If the leg rides up when you walk a mile, the inseam is too short for your build. Standard boxer briefs run about a 5-inch inseam; a 6 or 7 stays where you put it, and if you're over six feet, go long. Trunks, which are the same thing with a 3-inch leg, work better for shorter guys because they don't visually shorten the leg under thin trousers. Boxers breathe best and bunch worst under fitted pants. Pick by what you actually wear over them.

Check the waistband on the pair you've got on right now. If it's wavy instead of flat, or if it rolls over on itself by mid-morning, the elastic is finished and no amount of washing brings it back. That pair is yard-work underwear now.

Fabric follows the day. Cotton for a desk, since it's comfortable and cheap. A nylon or polyester blend, or merino, for the gym, hiking, and long drives, because cotton holds moisture and stays wet. Flatlock seams instead of raised ones if you're walking distance or riding a bike. That's the whole science.

Buy one pair before you buy six. Wear it for a full day, including sitting for three hours. If it moved, it's not the right cut for you, and the eight-pack would've been eight mistakes.

Socks: height, heel shape, and the correct size range

The sock that slides down under your heel is almost always a tube sock, a size too big, or both. A shaped heel, sometimes sold as a heel pocket or Y-heel, has a seam built to cup the back of your foot. A tube sock has no heel at all, so it rotates, bunches, and raises blisters on a long day in boots.

Sock sizes are shoe-size ranges and people ignore them. If you wear a 12 and you buy 9-11 because it was on the shelf, the heel lands under your arch and everything goes downhill.

Height, by use:

• Dress socks: mid-calf at minimum, over-the-calf if you want them to never fall. No skin when you cross your legs.
• Boots and work: crew height, so the cuff clears the boot top and the boot never touches your shin.
• Sneakers: crew or quarter. If you insist on no-shows, buy the ones with a silicone grip strip at the heel. The others are decorative.

Fabric, by use. Merino for boots, cold, and long days, because it manages sweat and doesn't smell after eight hours. Cotton for around the house and the office. Not cotton inside a hiking boot, ever. Polyester-nylon blends with a bit of spandex for the gym.

One fit detail people learn the hard way: a thick wool sock changes how a boot fits by about half a size. Try the boot with the sock you'll wear in it, not with the thin liner sock the store hands you.

Match dress socks to your trousers, not your shoes. A gray sock under gray flannel keeps the leg line unbroken. A white athletic sock under a suit breaks it into pieces.

Buy one model, in one color, and buy it again

This is the part that turns a drawer into a system. Pick one dress sock in navy, one in charcoal, one work crew in whatever color hides dirt, and buy multiples of the same model. Pairing laundry becomes a non-event. Losing one sock costs you one sock instead of a pair.

Fourteen pairs of everyday socks means you do laundry once a week without ever hitting the bottom of the drawer. Same math for underwear. That's not extravagance, it's how you stop wearing the ones with dead elastic.

Set a replacement date and keep it. Once a year, pull everything out, and anything with a thin heel, a wavy waistband, or a stretched collar goes in the rag bag or the trash. Yes, all of it still technically works. That's the trap.

Care, briefly

Heat kills elastic. Wash cool, dry on low or hang them. Mesh laundry bags keep socks from disappearing into the machine's corners, and they keep undershirts from stretching around the neck.

Skip fabric softener on athletic socks and synthetic underwear. It coats the fibers and cuts the moisture handling you paid for. If they're smelling stale, a vinegar rinse does more than softener ever will.

Nobody's ever going to compliment your socks. The win is that by 4 p.m. you've forgotten you're wearing any of it.