The lights dim, the audience falls quiet and looks to the stage. The conductor strides out, climbs the podium and surveys an ensemble of between 25 and 110 men and women in various states of anticipation. They sit with their instruments, often as many as two dozen different kinds.

The symphony orchestra is about to play, producing one of the richest and most complex musical experiences humans can perform.

The instruments range in size and pitch from the tiny piccolo (an abbreviated flute) to the grandest of the strings, the double bass (as big as a man and for which most airlines will charge an additional fare, should an artist refuse to check a priceless instrument as baggage). The players' duties will vary: A violinist may be busy throughout the show; someone else will wait for half an hour just to strum briefly on the harp and then wait some more.

Still, everyone on the stage has one thing in common. All are members of a symphony orchestra, an extraordinary invention that came out of central Europe in the late 1700s and has continued to evolve.

But orchestras are not uniform. Almost every generalization about the modern orchestra can be contradicted. There are large and small ones, orchestras that rely on a strong conductor and orchestras that conduct themselves, orchestras that specialize in music written before 1800 and those that play only the most far-reaching contemporary experiments.

Virgil Thomson, the American composer and critic, once described the orchestra as "primarily a string combo," with a marked emphasis on the violins (about 30 are in a typical group). He was mostly correct. Still, another American composer, Philip Glass, scored his three-act opera "Akhnaten" (1983) without a single violin. Nowadays, an orchestra can be anything at all and, as a result, anything can happen.

Regardless of what a contemporary composer might choose to do with the music-making facilities available in an orchestra, the grouping itself hasn't changed much in the last century. [See the diagram inside for the complement of musicians in Washington's own National Symphony Orchestra (NSO), a fairly representative ensemble and an increasingly fine one.] More than half of the musicians in an orchestra such as the NSO play stringed instruments -- violins, violas, cellos and doublebasses, to list them from high pitch and small size to low pitch and large size. All four evolved from the 400-year-old viol family, a collection of bowed instruments with names such as treble viol, tenor viol, bass viol and division viol, all played while they rested on or between a musician's legs.

The viol family was superseded about 1700 with early versions of the instruments we know today, all of them with four strings, all of them usually played with a bow that rubs a bundle of its own strings against the violin's strings to make them vibrate. The strings also may be plucked ("pizzicato"). Except for the cello, all can be played while standing (indeed, the six-foot-tall double bass demands either a high stool or a standing position).

In most orchestras, the positions occupied by each instrument are fairly standardized.

In general, the string instruments are in front, nearest the conductor, with violins on his or her left. This allows the violin's "sound-holes" to face the audience for greater projection. The violas, cellos and double basses are to the right, the bases behind the smaller cellos. The rest of the instruments are positioned toward the back of the stage.

Because most composers write more than one part for the violins to play at a time, that section usually is divided into first violins and second violins. Playing second violin does not necessarily mean playing "second fiddle;" the lead second violin is a choice job in any orchestra.

Still, the most prestigious position among the violins -- indeed, the most prestigious position in the orchestra other than conductor -- belongs to the concertmaster, the first of the first violins, who walks onstage just before the conductor, is accorded many honors and laden with many duties (such as indicating a choice of bowing and fingering to the other violins) and, in Great Britain, often is called simply "the leader."

Sometimes, the first and second violins are on opposite sides of the stage, playing stereophonically. Some conductors -- Leopold Stokowski (1882-1977) most notable among them -- have believed in regularly rearranging the orchestra's seating chart. Leonard Slatkin, the NSO's new conductor, has moved the timpani, or kettle drums, from the left rear with the other percussion instruments, where his predecessor Mstislav Rostropovich liked them, to the right rear.

No other group of instruments plays so large a part of a typical symphony as the strings, but the brass (horns, trumpets, trombones and a single tuba) and the woodwinds (flute, oboe, clarinet, bassoon and the very occasional saxophone) profoundly influence the overall sound.

The percussion family is in a class by itself. There are dozens of different hammered instruments from which a composer might choose, ranging from tiny bells and finger cymbals to timpani and even an anvil. Even the piano sometimes is said to be a percussion instrument, and some pianists play it that way.

The timpani, a set of resonant, thunderous, tuned drums, is considered "king" of the percussion instruments and was the only one used regularly in early symphonic works. Finally, there are some "occasional" instruments -- harp, piano, celesta and organ, among others.

As dominant as strings are in the modern orchestra, they were positively ubiquitous in earlier ensembles. Most of the more than 100 symphonies by Franz Joseph Haydn (1732-1809), sometimes known as the "father of the symphony," are scored overwhelmingly for strings, with only two oboes, a bassoon, two horns and a harpsichord to diversify the sound.

By the end of his life, Haydn had added two flutes, two clarinets, another bassoon, two trumpets and timpani to his standard mix, and this is fundamentally the same form of orchestra used by the next two great symphonic composers -- Wolfgang Amadeus Mozart (1756-91) in his most mature works and the young Ludwig van Beethoven (1770-1827).

Since those days, symphony orchestras have grown considerably in size, creating a richer, lusher sound. As a result, there has been debate about whether works from this early era should be played by a standard modern orchestra. While there can be little doubt that Haydn and Mozart were accustomed to hearing their works played by a much smaller ensemble than we are accustomed to today, it is impossible to say whether a larger sound would have displeased the composers.

Most orchestras compromise on this issue now and present the symphonies of Mozart and Haydn with a somewhat diminished string section -- with only 10 violins, say, instead of 30. Still, some groups, notably the Vienna and Berlin Philharmonics, continue to offer performances of early symphonic music with the full, proud, high-calorie luxuriance available only from a huge string section.

Incidentally, the word "philharmonic" is an interesting one. It derives from Greek and means "friendly to harmony." Considering that the emphasis on music's traditional rules of harmony has diminished quite a bit in the 20th century, it is not surprising that the term now is somewhat antiquated and mostly associated with older orchestras.

"Symphony" is another word that deserves clarification. Literally, it means a "sounding together," and only in North America is it considered a virtual synonym for orchestra, as in "The Chicago Symphony." In Europe, if the word "symphony" is used to describe a group of musicians, they inevitably are referred to as a "symphony orchestra" -- that is, an orchestra large and capable enough to play symphonies, which are musical compositions.

The symphony as a musical genre grew out of the sonata form in the mid-18th century. A sonota is like a symphony in having several contrasting movements but is written for a solo instrument, sometimes with accompaniment by another instrument. It was largely Haydn who expanded the sonata to the symphony.

Early on, a proper symphony was written in three movements (usually fast-slow-fast) but, over time, the strictures were both augmented and loosened, and the word "symphony," in this context, now seems to mean simply "a substantial work for a large orchestra."

Most of the best-known works in this genre are in four movements, but there are also numerous examples of symphonies in three movements (many of the early Haydn and Mozart works and two by Jean Sibelius), several important symphonies in one movement (by Sibelius, Arnold Schoenberg, Samuel Barber, Roy Harris and Allan Pettersson, among others), a few symphonies in two movements (Gustav Mahler's massive Symphony No. 8 and Anton Webern's sole contribution to the literature), the beloved "Pastoral" Symphony by Beethoven in five movements, symphonies in six movements (the Mahler Third), going all the way to 10 movements (the "Turangalila Symphony" by the late French composer Olivier Messiaen).

"A mass of images, remembrances and ideals comes instantly to mind when we hear the word symphony," the critic Michael Steinberg wrote in his book The Symphony: A Listener's Guide. "This mass, our idea of 'symphony,' has been shaped for us overwhelmingly by the nine symphonies that Beethoven composed across a quarter of a century."

There is no doubt that Beethoven changed the symphony forever. In his first two works in the idiom, he continued in the style of Haydn and Mozart (adding his own distinctive -- and underrated -- sense of humor to the blend). With the third symphony, the so-called "Eroica," he began to break the rules; a spacious performance of this symphony, observing all repeats, will take close to a full hour. The early symphonies of Haydn and Mozart, by comparison, last only about 10-15 minutes. Moreover, the "Eroica" is startlingly disjunct and dissonant by comparison with the music of the time.

In later works, Beethoven helped to perfect the concept of "program music" -- music intended to represent something beyond itself, like a story or a picture. His "Pastoral" Symphony, for example, is an idyllic depiction of a day in the country, complete with an impromptu thunderstorm. And then, in the Symphony No. 9, Beethoven added four vocal soloists and a full chorus for the grandest and most ambitious exercise in symphonic form that had ever been attempted.

The 19th century was filled with great symphonies -- by Franz Schubert, Felix Mendelssohn, Johannes Brahms, Antonin Dvorak and others -- but the form itself changed little for about 70 years. When it was pointed out to Brahms that the finale of his first symphony resembled the last movement of Beethoven's ninth, his response was direct and to the point -- "Any ass can hear that!"

Still, toward the end of the 19th century, the long, searching, subjective symphonies of Peter Ilich Tchaikovsky, Anton Bruckner and Mahler opened the field again. By the beginning of the present century, symphonies that lasted an hour were commonplace (only two of Mahler's 10 symphonies can possibly be played in less than that time), the orchestra was expanded (in the operas and symphonic poems of Richard Strauss and, most spectacularly, in Mahler's so-called "Symphony of a Thousand"), and the more deliberately "cosmic" and philosophical elements of the Beethoven Ninth were explored.

The seven symphonies of Sibelius (1865-1957) are especially interesting, as each of his mature works has almost nothing to do with any of its predecessors. Sibelius planned the most radical of these, the Symphony No. 4, as a protest against the music of his time; it was spare, succinct, deliberately austere in its utterance. "While other composers have served up champagne and cocktails," Sibelius once said, "I have given the world cold, clean water."

Since Sibelius, composers generally have followed their own instincts as to what makes a proper symphony. The 20th century has been rich in such hypotheses, resulting in the wiry and idiosyncratic works by Igor Stravinsky through the lush and mostly gentle symphonies by Ralph Vaughan Williams, to cite two nearly opposite styles.

The United States has had a particularly good century, with valuable symphonies by, among others, Aaron Copland, Virgil Thomson, Walter Piston, Howard Hanson, Samuel Barber, Roger Sessions, William Schuman, John Harbison and Ellen Taaffe Zwilich, the first woman to win the Pulitzer Prize in composition.

Some critics have suggested that the symphony may be finished as a musical form. While it is true that, for practical and economic reasons if nothing else, composers likely will find it easier to locate performances of works for smaller forces -- piano, choral group or chamber ensemble -- there has been strong interest in the symphony from today's younger generation of composers, including Oliver Knussen, Aaron Jay Kernis, Daron Hagen and Christopher Rouse. And, in the early '90s, a Nonesuch recording of a contemplative, prayerful symphony by a reclusive Polish composer named Henryk Mikolaj Gorecki (the "Symphony of Sorrowful Songs") became a smash hit worldwide.

Whether the symphony orchestra can survive as an economically viable entity also is open to question. Indeed, many smaller orchestras have already disbanded, as a result of rising costs and, in some cases, bitter labor disputes with unionized musicians.

Increasingly, we rely on the media -- radio, recordings, film -- to provide the music we listen to; live concerts are very expensive to mount and, often, to attend as well. It is probable that without increased federal funding (something that seems highly unlikely at the moment), many midlevel professional orchestras will cease to exist within the next quarter-century.

The great orchestras, of which there are perhaps 15 to 20 in the United States, will survive, as will the small, semi-professional groups founded by music lovers, largely for the sheer joy of playing. Those in the middle -- the hard-working, professional orchestras established in a different era to be a community's central source of music -- are most in jeopardy.

In the meantime, we can content ourselves with several weekly performances by the National Symphony Orchestra -- and visitors to New York, Chicago, Cleveland, Philadelphia, San Francisco, Baltimore and many other large American cities will want to hear the various ways groups there make music. Several times during the year, the NSO presents free concerts (often outdoors), and this may provide an informal and frugal introduction to classical music.

We have left the conductor for last -- some groups, such as the chamber ensemble Orpheus, leave conductors out entirely.

The conductor's is the most mysterious of musical talents. Who hasn't wondered what that person was really doing up there besides making funny faces and waving his arms? If Leonard Bernstein, who seemed caught in an ecstatic trance most of his time onstage, was a great conductor, what does that say for Pierre Boulez, who enjoys considerable acclaim but leads an orchestra with the brisk, dispassionate efficiency of a bank teller making change?

An ideal conductor combines the best virtues of shaman, athletic coach, psychologist and traffic cop -- planning the program, rehearsing the orchestra, inspiring the musicians to give their best, nurturing new talent and making sure that the various players' performances don't collide.

Composers specify the notes but often give few directions as to how fast to play or how loud. So the conductor must set tempos, keep the beat going, signal the musicians when to enter (usually necessary only in difficult music), emphasize particular aspects in the musical score, attempt to strike a balance between traditional interpretations and a possible desire to invent a new one.

Many great works will survive radically different interpretations. To choose one extreme contrast, Arturo Toscanini used to bring in Wagner's "Siegfried Idyll" at about 15 minutes, while Glenn Gould, the late pianist and occasional conductor, made a recording of the same piece that lasts 25 minutes -- more than half again as long!

Both performances are admirable, but the entire character of the piece is altered by the different tempos selected by the conductors. This is part of the reason that many record collectors prize not merely one but several performances of favorite pieces.

And so every new concert, no matter how familiar the works, should add to our understanding and knowledge of music. When you team a first-class conductor with eager and accomplished musicians and then let them play, the results can be exhilarating -- an aural experience like nothing else on the planet.

Horizon Discount For the Symphony

The National Symphony Orchestra is offering a 50 percent discount on tickets to this Friday evening's concert, in which Leonard Slatkin is to conduct several short works featuring seven soloists from the NSO. This will be an excellent opportunity to hear the special qualities of many of the instruments in the diagram on the opposite page.

All you need to do is ask for the Horizon discount when making the reservation. The performance, Friday, Dec. 13 at 8:30 p.m., is to be in the Kennedy Center Concert Hall.

Readers may purchase tickets at the Kennedy Center box office or by phone at 202-467-4600.

Anatomy of a Symphony Orchestra

All music, like all forms of sound, is produced by making air vibrate. The faster the vibration, the higher the pitch. Larger objects vibrate slower than small ones. Other factors influence tonal qualities that differentiate instruments.