You probably didn’t realize, when you put an ornate old picture frame next to the sleek Pottery Barn sofa in your living room, that you had your finger on the pulse of America’s design culture.

But you did.

What some interior designers call “transitional” decorating — artfully mixing contemporary pieces with vintage ones — is “the No. 1 featured style in magazines like House Beautiful, and in Elle Decor on a fine level, and in Cottage Living,” says interior designer Mallory Mathison.

It’s a trend that seems tailor-made for 2009. It works in any room and helps you get style mileage out of things you already own or buy secondhand. It’s also environmentally friendly and practical to use older items in otherwise contemporary rooms.

“There is something to be said about a well-made, 50-year-old piece still doing its job in your present home,” says designer Brian Patrick Flynn. “A lot of older things were just made better. But mixing those quality older pieces with something new gives them a fresh look.”

Another money-saver: The look can be easily tweaked and ages beautifully, unlike all-modern or all-traditional rooms. “If everything is brand new and matchy-matchy,” Flynn says, “your home will look like a catalog or a showroom and appear flat or dated.”

Designer Janine Carendi agrees: “Interiors that do not age well,” she says, “are those that are designed without any reference to other styles.”

Mixing old and new has its challenges, of course. But it can be done just as easily and more affordably than sticking with one style. “Contemporary is hard to pull off without looking cheap if you don’t have very fine things,” Mathison says. “And traditional can get old and staid if you don’t have very fine pieces also.”