Menahem Pressler is one of the world's most nervous pianists when he is playing with the Beaux Arts Trio. His fingers glide across the board with a sure touch and almost effortless ease, but his eyes dart back and forth between his partners, violinist Isidore Cohen and cellist Bernard Greenhouse, as though he expects one of them to drop his bow at any moment. For those who watch them in the Coolidge Auditorium of the Library of Congress, where the Beaux Arts Trio will be in residence through the first week of March, Pressler's anxieties are hard to understand; his trio is one of the greatest and best-known ensembles in the world. At the library, it always plays to a full house--one of the world's most sophisticated audiences--at a price of 25 cents per ticket.

Across town, in the Dumbarton Avenue United Methodist Church in Georgetown, the price of chamber music (which happens once a month) is higher--but still no more than a first-run movie. The attractions are not usually world-famous (though next month's attraction will be soprano Judith Raskin), but the programs are imaginatively chosen and the hall is almost ideal for chamber music, with seating in the round so that nobody in the audience is more than 50 feet from the players. Most performers in this series are local and still relatively unknown--like the Manchester String Quartet, a brilliant young group whose members are all members of the National Symphony. Next year, the whole season will feature NSO players.

For all their differences, the concerts at the Library of Congress and those in the Dumbarton Avenue Concert Series define the element that gives Washington's musical life its special flavor. Military music is strong here, with four of the world's great service bands, and the National Symphony and Washington Opera are building solid reputations. But the center of gravity for Washington's musical life, the factor that makes it unique, is the abundance and quality of chamber music, the music of intimacy, living in strange symbiosis with public institutions.

What's so different about chamber music? The basic difference is one of personnel: in chamber music, there is only one performer for each part, whereas an orchestral score will have 20 violins--or even 40--all playing the same notes. But this simple difference has many implications.

Not too long ago, chamber music was considered the interest of a small clique of effete snobs. It was the subject of odd little jokes. In the 1940s, for example, there was a Dixieland jazz group in New Orleans that called itself (with arch wit and unwitting accuracy) the Chamber Music Society of Lower Basin Street.

Sociologically, orchestral music evolved in the courts of 18th-century Europe, particularly in the Austro- Hungarian Empire. It still represents a sort of idealized microcosm of that empire, particularly its military establishment, with the conductor as emperor exercising an enlightened despotism and all of the orchestra's members (divided into sections with sub-chiefs) following his will without question.

Chamber music is older than orchestral music and rooted in domestic rather than ceremonial life. The forms it uses (sonata form, rondo, minuet, theme and variations, etc.) followed the same evolution as orchestral forms, but its spirit is different because it was composed primarily to be played in private homes, by amateurs, for their own enjoyment. Thus, it reflects the spirit of give- and-take among equals in an informal setting, while orchestral music represents the carefully drilled execution of orders from above by a group of highly skilled specialists who sacrifice their individuality. Sometimes parts of an orchestral piece approachthe spirit of chamber music (solo passages in Mahler's symphonies, for example) but they are exceptions to the rule.

Economically, a string quartet costs approximately four percent as much to perform as a composition for 100-piece orchestra. This may make chamber music the ideal form for a period of recession.

Psychologically, chamber music represents a balance of freedom, mutual respect and dedication to common goals, while the virtues pursued in orchestral music are chiefly discipline and technical skill.

The way chamber music works is illustrated by the opening of Schubert's "Trout" Quintet, composed for violin, viola, cello, double bass (very rare in chamber music) and piano. After a loud opening chord, the strings all fall silent, except for the double bass, which holds a low A, very softly, for 10 bars--thinking things over while it watches what is going on around it. The piano repeatedly suggests a topic of discussion--a glittering arpeggio running up through two octaves, just the sort of thing a piano would think of. The strings (except for the double bass, which is deep in thought) react with several bars of counter-suggestion--a real melody, which is very beautiful, much more slow-moving and less flashy.

Eventually, the piano ofers a compromise motif, attracting the attention of the viola, but it upsets the violin, which immediately reminds the piano of its original statement. The double bass reacts by dropping down from A to F natural (a note that does not fit comfortably in the key of A with which the movement opened). The cello, after several bars of silence, begins to echo the piano. As the first page ends (in the inexpensive Dover edition of Schubert's chamber music for piano and strings), two loose, temporary alliances have been formed: violin and cello one one side, piano and viola on the other, while the double bass stands aloof from this discussion, quietly playing to itself.

Although each of the players is carefully following instructions written down more than a century and a half ago, the feeling is one of individuals looking at a proposition, making their own decisions and freely taking positions on what is to be discussed.

In most American cities, musical life centers on the local symphony or chestra; there are some 1,500 of them enrolled in the American Symphony Orchestra League, whose headquarters are in Washington. In European capitals, the musical focus is usually an opera company heavily subsidized by the government. Washington is a latecomer-- nouveau-riche, though not particularly riche--in both these departments, with an orchestra barely half a century old and an opera company half that age. But since 1925, Washington has been a Mecca for small groups who want to play for small audiences. That was the year Elizabeth Sprague Coolidge gave the Library of Congress the 500-seat auditorium that bears her name. It is one of the world's great places to hear chamber music--a place to which, in fact, the world tunes in regularly via weekly broadcasts of Library of Congress concerts.

The concerts and broadcasts are subsidized by endowment funds, with other subsidies to commission new music. Besides offering great performances, the Library of Congress has brought into being some of the most notable music of the 20th century: string quartets by Bartok, Schoenberg and Prokofiev, Copland's "Appalachian Spring," Stravinsky's "Apollo" and Crumb's "Ancient Voices of Children," to name only a few. Performers who have made their American debuts at the Library range from Bartok to Rampal.

This theme has long been followed in other Washington institutions: free concerts of high-quality chamber music that would command double-digit ticket prices anywhere else. Strictly speaking, the Library of Congress tickets are nominal, not free, and when the attraction is a hot ensemble such as the Juilliard Quartet or the Beaux Arts Trio you have to wait in line for tickets at 8 on Monday morning outside of Jordan Kitt's music store on G St., N.W.

At some other public institutions, the concerts are completely free. The Phillips Collection and the National Gallery simply have seats available on a first-come, first-served basis.

The quality of the music has no relation to the low or nonexistent ticket price; these concerts (which are also broadcast) offer connoisseurs a chance to hear the stars of the future before they become well known. Performers who appeared at the Phillips before they became established include Glenn Gould and Jessye Norman, Carmen Balthrop, Santiago Rodriguez, Gary Graffman, and Ann Schein. The National Gallery offered the Juilliard Quartet long before it played at the Library of Congress, and it has presented the world premi,eres of compositions by such composers as Crumb, Milhaud and Ives. Particularly notable is the festival of American music it offers every spring--at about the same time that the Inter- American Music Festival (also free of charge) is happening at the Organization of American States. That festival has featured premi,res by virtually all of Latin America's leading composers and performances by many internationally known soloists and ensembles.

In the shadow of all this free chamber music, dozens of organizations (like the Dumbarton Avenue Series) have sprung up to stimulate and satisfy the city's appetite for chamber music, and several superb new auditoriums have been opened in the last few years--among them the Terrace Theater at the Kennedy Center, the Barns of Wolf Trap and the refurbished Armand Hammer Auditorium in the Corcoran Gallery. Chamber Music can be heard regularly at the National Institutes of Health, the Smithsonian, the National Academy of Sciences, the World Bank (which will open a new auditorium later this year) enough other places to fill pages of listings. Specialized performing groups range from the Twentieth Century Consort, which plays the music of today, to the Folger Consort, which offers music of the Renaissance and Middle Ages. These programs are not subsidized (though an endowment fund, named in honor of Abe Fortas, is being raised for the Terrace concerts), and the tickets are not free, but prices are generally low. They have to be in a city where the competition regularly includes the Juilliard Quartet playing for two bits per ticket.

In Washington today, chamber music is the most democratic form of serious entertainment we have-- poor people's music, or at least music that unites its aficionados without regard for economic status. Structurally, it is the music of total equality, of free-flowing dialogue in which discord resolves itself into harmony. Not a bad kind of music for the capital city of a nation that considers itself the standard-bearer of democracy.