THE BASEMENT is to the American what the bazaar is to an Arab or his club is to an Englishman: a stage for his essential social self. But call it rec room, den or family room -- the basement is now threatened with a phase-out as industry cuts back to build a no-frills home on a concrete slab.

"Save the basement!" the sticker should read. Such a great American institution must not be allowed to go the way of The Washington Star, the Department of Health, Education and Welfare or the blue whale. Builders must be made aware of the irreversible damage they cause each time they build a home without a basement.

In the living room and in the dining room, we give in to the pressures of society; from scatter rugs to embossed ceilings, we bow to the authority of Better Homes and Gardens. We ask our children to stay off the sofa, and we have chairs too precious to sit on.

The basement is the alternative to upstairs; it is the wide-open space a homeowner stakes out for himself.

It is the home's frontier -- an opportunity for relaxed living and self-assertion, a parade ground for manual skills and private fantasy.

In the Old World, the basement is the ante-chamber to oblivion. Bad children are threatened with banishment to the ill-lit, musty cellar that was an air-raid shelter during the war. Mice scurry among stacks of old papers, and up and down an armchair that grandfather broke in his days of youthful passion.

The American basement, however, is the incipient future. It's Dad's favorite project, the children's natural playroom. Teenagers demand it as their birthright.

When offering an unfinished basement, the real estate salesman has us in the palm of his hand. Staring at the cinder block walls, we see ourselves ensconced in the quiet elegance of Beacon Hill maple or brooding in the midst of dark Tudor oak. Our dreams may call for palace white or barnboard rustic, a tone of prairie wheat or ocean mist. We may set ourselves up as a big-game hunter or an oriental potentate, an electronic wizard or a medieval chatelain.

The basement offers a market for technology and a focus on comfort. No other part of the house is as cunningly lighted and insulated, as padded with foam rubber cushions and built-in couches, and as lavishly supplied with audio equipment.

Romantics cite the scent and touch of wood, and pay exorbitant prices for tongue-and-groove pine planks. Some of them go as far as to panel the walls with second-hand wood they sanded and refinished. But for the great practical majority, the choice is the richly simulated woodgrain of 4 x 8 sheets of Masonite or Acrytuff. "Panel Today, Party Tonight," the ad promises. All that you need to know is in a $2.95 guide, and the instructions are of course in a simple, easy-to-follow language. And if there is a crisis, a neighbor is always glad to be consulted.

There are people want their basement to be like the rest of the house, and use sheetrock for wall and ceiling. "Paneling limits you to one thing: a basement," one of them argues. "But with white walls, our basement is an open vista -- like a blank page in a typewriter."

But even for those who don't want their basement to be a basement, the stairs down from the kitchen lead to one part of the house where informality is de rigueur. No matter what the occasion, a necktie is out of place and shoes seem superfluous. "But it's only the basement," my 9-year-old son complains indignantly after being told to pick up his toys off the floor. "It's not supposed to be neat here."

A proper basement has no oak parquet or Anatolian kelim to worry about. Even if it's called the family room, that poster of a lady wearing only spaghetti is just right. Tacked to the wall, of course. The rec room is not a rec room without E-Z Kare, wipe-'em-clean virgin vinyl for the walls and soil-resistant propylene for carpenting.

We mean the basement when we say down-to-earth. We must dig in and preserve it.