St. Louis ? T-shirts.

So simple, you hardly notice them when they’re worn. You just slip one on and chill.

Rarely does anyone judge a T-shirt unless it’s somehow ill-fitting; otherwise, it’s just seen and accepted.

First used by military men as undergarments, today’s T-shirts are the ubiquitous piece in everyone’s wardrobe.

Dressing up? A gathered-front, cap-sleeved tee will probably do the trick. Fishing? A ribbed tank tee with blue-jean shorts may cast you in the best light. At the office? A crisp white one under a jacket or cardigan is sweetly chic.

How can such a nondescript, cotton top attract such a following among the young and old, the prude and crude, the famous and fanatical?

“It’s basic and can be worn with anything, and I think Hollywood has brought it back to the market with all of the stars wearing them, such as Cameron Diaz,” says Laura Coughlin, store manager of Charlotte Russe at St. Louis Mills.

While, indeed, a typical tee is made of cotton, has a round (or crew) neck and short sleeves, there are many variations of it. More than anything, Coughlin says, a T-shirt connotes comfort.

But she offers a word of advice to those ready to slip into its stretchy confines: “Make sure it’s not too small for your body type,” she warns. No better way to ruin a look of comfort than drawing attention to everything you don’t want to highlight underneath.

Others agree.

“The No. 1 question we get is, ‘Do I look like I have back fat in this?’ ” says Drea Ranek, co-owner of Lusso in Clayton, Mo. “So it’s important to buy the right size and not get it too small.”

Big isn’t always better, either. “Oversized ones can look sloppy,” Ranek adds.

And trend alert: Ranek says such trends as wearing your sleeves rolled up (like we did in middle school) or twisting the bottom front in a knot just “aren’t ‘in’ right now.”

In fact, T-shirts are at their best, she and others say, when they’re simply left alone.

For women, Ranek says the crewneck, short-sleeved T-shirt is still “the one that everyone goes for.”

And while she insists that “Midwestern boys” like a little space in their T-shirts, Ranek says, not that much.

“The move is toward a slimmer shirt, not the oversized, extra-extra large that they were doing for so long.”

Ringer tees (the kind with contrasting fabric at the neck and arms) have made a comeback, too, as have shirts with slogans and themes.

“The novelty T-shirt thing is really opening up for guys,” Ranek adds. “At first, guys thought they needed to wear a solid black, gray or blue T-shirt. Now, they’ll wear ones with more personality.”

Among the top T-shirt trends? Layering them, be it short over long or prints under solids, notes Old Navy spokesperson Alexandra Cohan.

“It’s a way to have fun dressing in the summer,” she adds. “It’s about personal style and what makes you feel your best.”

Cohan mentions other trends as well.

“I am seeing people knotting T-shirts in back for more of a fitted style,” she says.

Men, she says, are rallying to Old Navy’s newest fit, the vintage fit, that is “thinner through the waist and fitted in the shoulders.”

Of course, T-shirts come dipped in all sort of shades this summer, but all-white ones remain among the most popular and versatile of the bunch.

“You can wear it with skirts and high heels or jeans and flip flops. It’s so easy,” Lusso’s Ranek reiterates about the simple basic cotton tee. “They’re so comfortable, it’s hard to want to wear anything else.”