MOZART AND Shubert did it with their friends for pleasure. Members of the nobility hired other people to do it for them in their homes. Johann Sebastian Bach did it with members of his numerous and highly talented family.

Today, it is being done more widely, more intensively, more promiscuously than ever before. In Washington, it is done in libraries and museums, on campuses and in the Kennedy Center, and at high noon on weekdays in a shocking number of downtown churches. Abe Fortes does it with three friends, and David Lloyd Kreeger does it in his home with some of the world's greatest professionals.

It is, of course, chamber music -- the music of intimacy. The sounds weave together like brightly colored threads in a tapestry. It may be the familiar flavor of a string quartet or it can be something much more exotic: a flute blending with a violin, a double bass chuckling at a piano's fantasies, a cymbal accenting what has just been said by an oboe. Whatever the combination, it is an encounter of individuals, a dialouge among equals. Its sound is delicate, but it is roaring across America and nowhere more strongly than in Washington.

Chamber music -- essentially, music in which there is only one voice or instrument to each part -- is older than orchestral music or opera, and it has always had a small but fervent following. Until Beethoven's time, it was primarily a cottage industry, music composed to be performed in private homes by amateurs, to be enjoyed as much by the performers as by the audience. Often it still is. But in its own small-scale way, it is becoming big business.

More than 75 percent of the 350 performing groups that belong to the national organiztion called Chamber Music America have come into existence within the last 10 years, and only a handful were established before 1965. Ben Dunham, the executive director of CMA, winces when you ask him about the chamber-music boom. "Please don't use that word," he says. "Let's call it a crescendo."

In Washington, you can hear chamber music at the National Bureau of Standards, NIH and the National Academy of Sciences; at the First Baptist Church, the Dumbarton United Methodist Church and a dozen others; at most of the museums in the Smithsonian complex, the Corcoran, the Phillips Collection, the YWCA, the World Bank, the Organization of American States, the National Gallery. A complete list would fill pages. The performers range from internationally famous ensembles like the Julliard and Tokyo Quartets, the Stern, Rose, Istomin Trio and the Chamber Music Society of Lincoln Center to the relatively unknown but carefully chosen performers at the Phillips Collection. Ticket prices range from double digits to zero, and -- on Washington more than anywhere else -- the price tag often has little relation to the quality of what will be heard.

Chamber music is free at the Phillips Collection and the National Gallery. At the Library of Congress, tickets cost 25 cents and you buy them at Jordan Kitt's on G Street NW. If the program and performers are especially appealing (as they frequently are), you wait in line outside Jordan Kitt's at 8 o'clock on Monday morning. If the Julliard Quartet is playing, at it is now, the line begins to form at around 6:30.

The Library of Congress offers the oldest and most distinguised free (or almost free) chamber music series in Washington, supported by income from private endowments. It has been the scene of hundreds of world premieres, including those of Bartok's Fifth Quartet, Copland's "Appalachian Spring," Elliott Carter's Duo, Crumb's "Ancient Voices of Children," "Stravinsky's "Appolo" and Schoenberg's Fourth Quartet. Noted artists who made their American debut there include Rudolf Serkin, Jean-Pierre Rampal and Bela Bartok, who gave a series of concerts in 1940 with violinist Joseph Szigeti.

"It was really the library that got Bartok over to the United States," says Donald Leavitt, who is in charge of the program. The library considers its series " the national chamber music platform," and shares it with the rest of the United States through a weekly broadcast. (The Phillips Collection and National Gallery programs are also broadcast.) In recent years, Leavitt says, "there has been some talk about raising the ticket price -- perhaps to a dollar -- but I have resisted it. Tickets were a quarter when I was a kid, and they will remain a quarter as long as I have anything to say about it."

Money is not the bottom line for most people who play chamber music, and the relatively modest economics may help to explain its growth. "It takes a lot of money to move a symphony orchestra," says Jan Kendall of the Washington Performing Arts Society, which books orchestras and soloists as well as chamber emsembles into Washington and subsidizes a lot of grass-roots activity by local performers. "Even if you sell out a big hall at top prices, it's almost impossible to break even with a visiting orchestra."

But the expense of orchestras hardly begins to explain the popularity of chamber music. Part of its growth may be due to the new orientation of listeners, who tend to hear music mostly in their homes on radios and hi-fi systems. The Chicago Symphony can be an impressive visitor in an average living room, but it tends to make the space a crowded, while a string quartet fits in nicley. "I began to notice a few years ago that when I got orchestral music on my raido I would turn it down, but when chamber music came on, I would turn it up," says "Connie Simmer, co-founder of the Dumbarton Concert Series, one of the newest and most interesting in Washington.

CMA's Ben Dunham believes that television helps to increase the chamber music audience, as it has done for opera and ballet. "Chamber music is much easier to get on the small screen than orchestral music," he says, "becuse of its smaller scale. You can show the total picture without losing detail, as you do when the total picture includes 100 players or more." Leon Fleisher of the Theater Chamber Players says that chamber music, in a sense, can be called "musical soap opera" -- not in terms of the quality of its material but because "it gives the audience a chance to become aware of the powerful but subtle interplay between a few people. Dealing with an orchestra, you have a great mass and it's harder to be aware of this interplay. Chamber music is really about interpersonal relationships, which are of interest to everyone.

Sociologically, chamber music is more of a 20th-century phenomenon than orchestral music. Composers seem to be producing much more, proportionately, now than in the classical and romantic periods. Part of the reason may be economic; it is easier to have new music performed if it requires only a few players than if 100 or more must go through long, expensive rehearsals. But it also reflects the individuality of our time in away that orchestral music does not.

Orchestral music was born and developed chiefly under the Austro-Hungarian Empire, and its organization, with a leader and sub-leaders of homeogeneous groups united in a single entity, reflects both the public and the military life of that period. Chamber music also began to develop in the same environment, but its texture reflects the free interplay of private citizens.

Some of the most interesting young musicians in America live in Washington -- violinist Jody Gatewood, violinist Miles Hoffman, solo pianist Barbro Dahlman, piano accompanist Frank Conlon and clarinetist Loren Kitt, to name only a handful -- and the musical scene here is active and rewarding enough to keep them in town, as it might not have been a generation ago.

This is not entirely an accident. "Both Leah Johnson and I got into this because artists were leaving Washington," says Connie Zimmer of the Dumbarton Concert Series."They were leaving because they thought they weren't being taken care of, and we thought one way to take care of them was to open another performing space, provide an audience, give them a chance for critical review and, of course, pay them. We ask for a $2 donation, but if someone wants to walk in we won't stop them. We do pay and, from what I understand, we pay in the middle to upper bracket: $400 per group per performance."

The Dumbarton series is a small part of a large picture. Few Washington musicians can make a living entirely on chamber music, but in recent years chamber music activities have begun to give some of them a supplement to what they can earn teaching or playing in an orchestra. One permanent group -- The National Symphony Wind Soloists -- was begun in 1958, and two others -- the Smithsonian Chamber Players -- date from the late '60s, but most groups were started in the '70s, including the Contemporary Music Forum (1973), the American Camerata for New Music (1974), the Twentieth Century Consort (1975), A Newe Jewell, the Folger Consort and the Romantic Chamber Ensemble in the last few years.

Along the way, Washington groups have profited from strong institutional support. The most prominent sponsor is the Smithsonian, which began to give concerts on its old musical instruments in the '60s with a group that developed into the Smithsonian Chamber Players, specialists in the performance of 18th-century music on 18th-century instruments. "We felt," says James Morris, director of the Division of Performing Arts, "that instruments should not just be displayed in glass cases -- they should be played." From that simple beginning has sprung an enormous program that includes more than a dozen visiting groups and three resident ensembles with a repertoire that spans the three centuries from the baroque period to contemporary music.

Besides the Chamber Players, these groups include the 20th Century Consort and the Romantic Chamber Ensemble, which are actually the same musicians playing under two different names in two different kinds of repertoire. With its vast resources, the Smithsonian has specialized halls for each of these groups, with an atmosphere appropriate to the music: the Hall of Musical Instruments for 18th-century works, the Renwick Gallery for the Romantic period and the Hirshhorn for modern. All three groups also participate in a perennially sold-out series called "Three Centuries of Chamber Music" at the Baird Auditorium, which also hosts many nonresident performing groups each year.

Washington's chamber-music boom has been particularly impressive in repertoires outside of the standard 18th-and 19th-century classics, with two relatively new groups (The Folger Consort and A Newe Jewell) specializing in early music and three groups that explore various approaches to contemporary music.

The eldest of these modernist ensembles, the Contemporary Music Forum, is also the most experimental, emphasizing very recent work and the work of Washington composers. The American Camerata occupies the borderline chamber ensemble and chamber orchestra (including concertos, for example, in its repertoire). The Twentieth Century Consort devotes much of its energy to the established classics of our time, includes "durability" among its criteria for selecting repertoire, and has attracted an enthusiastic following with its virtuoso performances.

Before any of these, the Theater Chamber Players were already pushing the cause of modern music in Washington and probably helped to prepare the audience for the others. "We are devoting a lot of our time to less familiar repertoire," says co-founder and music director Leon Fleisher. "We feel it is very important to play the music of our time to show people the threads from the past out of which the present-day tapestry is woven." They might attract larger audiences if they would concentrate, as they certainly could, on the classical chamber music "top 40." This would add to personal enrichment but contribute much less to the enrichment of the city's musical life.

In the future, that life is likely to grow richer: Washington has not seen the end of new ensembles. Pianist Alan Mandel and baritone Jerome Barry, two local musicians with international careers, are now looking around for kindred souls to form another group. A consortium of Washington musicians and ensembles, under the collective name, "Millennium," held a chamber music festival last summer on Martha's Vineyard and is exploring other outlets for Washington talent. Baroque oboist Stanley King has just started a new group, Wondrous Machine, which will introduce itself with an all-Telemann program on original instruments April 20 and 21, after King returns from his current gig at La Piccola Scala in Milan.