When it comes to Mozart, the key word for a festival seems to be "mostly." The man composed 41 symphonies, 27 piano concertos and 23 string quartets -- including a handful that are a smash at the box office and many more that remain practically unknown although recordings have made them easily accessible.

Haydn is an even more extreme case, with 104 symphonies, 82 string quartets and 31 piano trios -- not to mention 125 trios involving an obsolete instrument called the baryton -- of which a half-dozen have been performed in Washington.

Tchaikowsky? The public attitude on Tchaikovsky is accurately conveyed in the fabled answer given by a schoolboy to an examination question: "Tchaikovsky composed three symphonies: Nos. Four, Five and Six." If Haydn and Mozart produced too much for a festival that can be labeled "complete," Tchaikovsky didn't produce quite enough.

So who's left? Obviously, Beethoven. His nine symphonies, five piano concertos, one violin concerto and one triple concerto for violin, cello and piano provide just the right amount of material for an orchestra to tackle in a series of weekend concerts. There's plenty of variety and practically everything on the menu is certified prime quality. It's an idea whose time has come -- again and again. The complete Beethoven cycle, which the National Symphony Orchestra launches this weekend, seems destined to become an annual event. But it's not the first this year. All 32 of the piano sonatas have already been performed in the Terrace Theater at the Kennedy Center.

The Juilliard Quartet is halfway through a complete cycle of the string quartets, which will resume in a few weeks at the Library of Congress, where it's being recorded for CBS Records.

Is this too much Beethoven? Hard-core fans will tell you there can never be too much. Those who disagree can go and find themselves another festival. For instance, there's a Brahms chamber music festival planned for February at the Wolf Trap Barns, featuring the winners of a Brahms competition for young singers and performers on the usual chamber music instruments. There's also a complete cycle of all Bach's music (including more than 200 cantatas) slowly proceeding under the auspices of the Washington Bach Consort. The whole project is expected to take 10 years, more or less, but what's the hurry?

As for Beethoven, he no longer occupies the lonely heights of unique eminence in which music-lovers of a generation or two ago held him. But he remains one of music's unique giants; his figure dominates the music of a whole century. Awareness of his presence can be heard in virtually all the composers who followed -- almost obsessively in Brahms, ambivalently in Bartok, Schoenberg and Mahler; rebelliously in Michael Tippett, who cast one of his own symphonies into a form that amounts to a quarrel with Beethoven's Ninth. For those who want to know what has happened in music since 1800, Beethoven is the inevitable starting point.

Along with the symphonies and concertos, the NSO festival will include a variety of overtures, some ballet music and the interesting little Choral Fantasy -- a sort of preliminary sketch for the last movement of the Ninth Symphony. It begins this Friday with the "Eroica" Symphony as the major attraction, and continues every Friday and Saturday night through September 11.