Those who have become addicted to Birkenstock sandals in spring and summer are familiar with the famous contoured footbed made from layers of natural cork and jute fibers. They are the comfortable sandals both men and women stare at in the store windows. Birkenstocks are made to provide cushioning for the foot but also to allow the foot to relax and breathe.

Even though Birkenstocks cost a bit more than other brands, they have been traditionally good sellers in the Rockies, the Pacific Northwest and even New England.Now as they celebrate their 30th anniversary of business - Margot Frazier brought them from Germany in 1966 - they are spreading their roots across the country. According to "Footwear News," a footwear-industry publication, students everywhere are adding Birkenstocks to their wardrobes. "Birkenstock is accelerating. Clogs and sandals are continuing to grow."

This is the deal: Birkenstocks are not just for summer anymore.

With the recent addition of Birkenstock's fully enclosed Footprints shoes and Classic clogs, plus the general acceptance of the "sandals-with-socks" look, Birkenstock has become a year-round shoe.

Retailers across the country are attributing the seasonal increase in Birkenstock sales to the company's new lines, to the addition of rugged, oiled and buffalo leather materials and to the new Birkenstock sock styles. Birkenstock's casual comfort is now available in sandals, shoes, clogs and socks in 300 styles and color combinations.

Because wearing socks with sandals has recently become a trend, even high school students have picked up on it. Birkenstocks seem to be on fire. It doesn't hurt that casual Fridays have become almost institutionalized at many businesses across the country, meaning that most people are into a casual look for everyday wear.

Footprints feature fully lined, top-grain leather uppers hand-stitched to polyurethane soles. The line, introduced in '94, was expanded last year to include open-toed, stitched-upper sandals.

This year, two highly popular styles, the "Madera" closed-toe fisherman and the "Oklahoma" clog, were added and have demonstrated themselves to be the company's strongest Footprint styles.

Footprint shoes, called by the company a marriage of European style and Birkenstock comfort, offer a removable, contoured cork footbed, combining the legendary comfort of Classic Birkenstock sandals with the styling of a conventional shoe and convenience of reparability and replacement. The footbed supports the arches, cushions the heels and gives the toes room to move naturally.

All Birkenstocks are made of thick, quality leather, suede and nubuck, plus a Birkenstock synthetic material called Birko-flor. Makers of Birkenstock say that asphalt, concrete and pavement are the inspiration for their footbed. Creating a natural environment for the foot mimics the support and freedom anyone could enjoy if the world were not paved.

Styles are available in unisex sizes with a suggested retail price range from $120 for the Oklahoma clog to $165 for the Madera fisherman and $175-$185 for the fully enclosed lace-up Derby and Memphis, Pasadena wallaby and Trent monk strap-style shoes.

Footprints are notable for their detail, from the finely crafted buckles to the hand-stitched leather, and they are dyed all the way through for long life.

Birkenstock socks were added in 1994 to meet consumer demand.

Cotton styles have a suggested retail price ranging from $7.50 for the Shorty Crew, to $9.50 for the speckle Sabatini, Softball crew and colorful Rugby, to $11.50 for the striped Shaker. The Eco Friendly Boot Sock made of Fortrel Eco-Spun fibers has a suggested retail price of $9.50.

In the Salt Lake area, Birken-stocks can be found at Clog Corner in Trolley Square or Kirkham's Outdoor Products, 3125 South State Street. Kelly Stewart, an assistant manager at Clog Corner, says she sells Birkenstocks to "pretty much everyone - older people buy them because of orthopedic problems and younger people buy them for comfort, and because they last a long time."

The new Footprints collection is not available locally yet, but it will be included in the winter stock. Anyone wishing a free Birkenstock catalog may call 1-800-597-3338.

Only one rule need be observed: please don't wear Birkenstocks to either a wedding or the opera.