Flip-flops are like dirty gym shoes, says Miami footwear designer Donald J Pliner.

“They’re not the nicest-looking things, but they’re so easy to wear, so comfortable. You don’t have to think about it. You just slip your feet into them and go,” he says.

“Basically, people are lazy. No wonder flip-flops are popular.”

The casual rubber slip-ons, so suited to beachside living, have always been a Florida favorite. But flip-flops have moved way beyond casual in recent seasons particularly the women’s styles. And they’re no longer made just of rubber.

Now they come in leather, denim or rope, with heels or platform soles, and decorated with buckles, sequins or crystals.

The single, identifying feature that remains unchanged is the double strap that comes between the first and second toes. That thong is what separates flip-flops (no matter how embellished) from all other sandals.

Flip-flops are not the invention of American surf shops, although their association with those establishments might lead to that conclusion. Rather, they are copied from Japanese zori sandals made with sponge-rubber soles and a thong between the first two toes.

It was back in the late 1990s that flip-flops made the leap from beach to city sidewalk, says Jen Mooney, fashion editor at Footwear News, a trade publication.

“In New York a few years ago, everyone started wearing cheap rubber flip-flops. They look great, they’re easy to wear, they’re really inexpensive so why not? They became a huge trend overnight,” she says.

Designers took note, and for a couple of seasons sent their models down the runway wearing flip-flops even with ball gowns.

“They give an outfit a funky edge, a certain attitude, casual but chic,” says Mooney. “Almost every company does them now.”

That includes such style leaders as Ferragamo, Prada and Gucci, and trendy newcomers such as Kate Spade, Faryl Robin and Lisa Nadine. Even Chanel has put its famous double C’s on flip-flops, which sell for a cool $405.

With such a high-priced stamp of approval, there’s no doubt about it: The humble beach thong has morphed into a high-fashion phenomenon.

Pliner designs flip-flops with supportive orthotic footbeds, platform soles, 2-inch hourglass heels and straps of brightly colored suede or metallic leather.

They’re beautiful and sensuous, he says. And they go with everything.

“Flip-flops are the easiest solution to summer footwear.”