Just when women had shaken off the last vestiges of anxiety over the skinny jean, the sado-fashionistic denim masterminds came up with the jegging -- a creation so skin-tight, so utterly form-fitting, it makes the skinny jean look almost baggy in comparison.

The jegging: part jeans, part legging. Entirely responsible for any recent runs on Weight Watchers.

Though denim leggings appeared at fashion-forward shops last fall, it's only this spring that they slipped into the mainstream, available in upscale boutiques for upward of $200, mall stores such as J Crew, and in discount department stores for less than $20.

Young Hollywood celebrities, including Blake Lively, Lauren Conrad, Paris Hilton and Jessica Alba, have all been spotted and photographed in jeggings.

And they're not going anywhere in a hurry.

"Last fall, people couldn't make the distinction between the skinny jean and this one, which is like a snap-on tight, tight like a glove," says Baltimore stylist Pascale Lemaire. "It's really caught on for spring. You're really seeing it out and about."

The advantage to the denim legging, fans say, is comfort. Fabulously stretchy and usually lightweight, they've got more give than skinny jeans. But because they're more substantial than leggings, they're a tad more forgiving.

They come in two basic styles. There's the legging done in a denim-looking knit, sometimes with top-stitching and a faux zipper to make them look more like jeans. Then there is the real jeans variety, crafted of extremely stretchy denim and often with real zippers and waistbands.

As happened when the skinny jeans debuted, a lot of women, those who haven't walked a runway recently, are a little leery of the painted-on look, wondering if jeggings are for them. And Lemaire thinks their concerns are more than justified.

On the street, she's been seeing a lot of unfortunate jegging faux pas, and the stylist thinks if women dare to wear these things, they need to abide by some basic ground rules.

First and foremost: Hide the Jeggings Butt.

If Lemaire can see the outline of your underwear, you're doing it wrong.

"If you're going to wear a legging, your butt should be covered," she says. "No one needs to see your butt in something that tight. Leggings should always be an underpinning, never an outer-garment."

For spring, Lemaire likes denim leggings with long, loose tunics that hit mid-thigh or under a very short dress. It's essentially a way to wear something too short to wear alone, she says.

Another rule: Don't pair them with tight tops. Lemaire recommends a top with a bit of an A-line cut to create a more balanced silhouette.

The jegging isn't made for women at either end of the weight spectrum.

Lemaire recommends that women who want denim leggings and aren't a size 2 stay away from the lighter washes and the trendier styles with acid wash or a tie-dye look. They should look for a pair in a more flattering dark blue or black.

For shoes, flats will work, but Lemaire prefers jeggings with a heel, a wedge or a shoe-boot, which keep the slim and sleek leg line.

Buyer Courtney Cannata thinks they work with everything from pumps to flip-flops.

Though it might seem like a look for those in their 20s, Lemaire says that doesn't have to be the case.

"It's definitely a younger trend," she says. "But if women are in their 50s or beyond in great shape, they should go for it."