SOMETIMES IT'S good news when lights burn late at the Pentagon. Among the men and women who stream into the building after sundown to staff the war room and monitor military communications are a lighthearted bunch who come to . . . dance.

Yes, dance. For more than 20 years the Pentagon concourse has echoed to the sound of country music and the square dance caller's commands: "Shoot the star . . . Slip the clutch . . . Box the gnat . . . Explode family!"

While the language may bemuse the passerby or passing spy, the code's an utterly innocent one. It keeps the scores of dancers whirling and twirling in intricate patterns reminiscent of Busby Berkeley, with only an occasional laughing tangle when someone confuses "Dive thru" with "Cross trail thru."

"Square dancing is friendship set to music," says a young man who drops out of a square to greet a newcomer. Like many members of the Bachelor & Bachelorette Square Dance Club, he's an unmarried government employee who despises the meat-market singles scene of nightclub cruising, personals ads and one-night stands.

"People need touching, not handling," adds Jean Lautzenheiser, a music teacher. "This is a place where single people get a lot of good safe touching." The friendly hug is institutionalized among square dancers: The move is called a "yellow rock," and you're likely to get one laid on you at any moment.

Square dancing's also good aerobic non-impact exercise. "My husband and I go square dancing five nights a week," says member Mickey Roberts. "He's 87. Never mind how old I am."

Friendliness and shared traditional values are the only social lubricants at a B & B affair, commented longtime member Jim Frain, "because you can't drink and follow the steps. You'd stumble around and spoil it for yourself and everybody else."

Actually you can quite easily get all bollixed up without touching a drop. There are three levels of expertise in "modern," or "western" square dancing, with 106 steps recognized and standardized by the American Square Dance Society. The B & B club has lesson sessions every Tuesday and dances every Thursday, whatever the season or the world situation.

Once you've mastered a step you never have to worry about it again. The same movements are called by the same name by all of the hundreds of square dance clubs in the Washington-Baltimore area, several of which are gay/lesbian, and the 10,000 clubs in North America. Not to mention perhaps a thousand square-dance clubs all over the world (including 13 in Saudi Arabia but none in Iraq).

"You can find yourself dancing with a club where the calls are the only English words you'll hear," says B & B president Robert Wood. "The calls just don't translate."

Indeed they don't: Imagine trying to render "Flutterwheel family" in Turkish (four square-dance clubs) "Spin chain the gears" in Japanese (34 clubs), or "Dixie-style to an ocean wave" in Swedish (120 clubs).

There's hardly a major city in the world that doesn't have at least one square-dance club, which takes a lot of the dreariness out of traveling on business. Making visitors welcome is not only a universal practice, it's written into the club charters. This aggressive friendliness includes formal raids on other clubs: One or more eight-person "squares" from one club will turn up at a dance and "steal" the hosts' banner, which can only be retrived by mounting a counter-raid.

Success hasn't spoiled this particular singles club, which was founded in 1967. Although at any given time about 15 percent of the members will be married to each other, the rule is that they may dance with anyone but their spouses.

"It keeps cliques from forming," a married member says, smiling.

"And it keeps the juices flowing," adds his wife, winking.

BACHELOR & BACHELORETTE SQUARE DANCE CLUB -- Weekly two-hour lessons begin Tuesday at 7:30 at the Pentagon; $45 for 15 lessons. Dances are held Thursdays at the Lincolnia Senior Center, 4710 N. Chambliss St., Alexandria. For information, call Robert Wood at 893-9591 or Pete Peterson at 435-0556.

THE WASHINGTON AREA SQUARE DANCE Cooperative Association has information on the nearly 150 square dance clubs in the metropolitan area. This fall, 30 callers will be offering classes. For information call Jerry Reilly at 560-1719.