Turning pages for a musical soloist is thankless work. Do it perfectly and you may get a small nod for menial labor. Make a mistake and you're the office boy who forgot to fax the one page on which the whole multinational deal depended. And there are so many small perfections to be mastered: the quiet turn during soft passages, the efficient turn-and-crease during fast ones, the return turn when the music repeats. And one page at a time please, and don't tear the music.

It's easy to sympathize with one of Beethoven's most famous page turners, the conductor Ignaz Seyfried, who left a harrowing description of turning for the master: "At the most on one page or the other were a few Egyptian hieroglyphs wholly unintelligible to me, scribbled down to serve as clues for him; for he played nearly all of the solo part from memory, since, as was so often the case, he had not had time to put it all down on paper."

Or had no need to, and Beethoven's piano concertos are works that resist simple ideas of completion, closure and finality. Perhaps the composer hadn't had time to finish the piano part. More likely, as a performer of his own work, he "finished" it only in the process of performance.

That's what makes the National Symphony Orchestra's Beethoven Festival, which officially began last night with a performance by the Ying Quartet and resumes tonight with Piano Concertos Nos. 1 and 5, such an interesting proposition. Music Director Leonard Slatkin will lead performances of all the major Beethoven concertos--five complete ones for piano, one full score for violin, the Triple Concerto for Violin, Cello and Piano, and various oddities, miscellaneous bits and short pieces for solo instruments and orchestra--over the next two weekends.

The orchestra headlines the event as "The Consummate Concerto: 1999 Beethoven Festival Features Every Beethoven Composition for Solo Instrument and Orchestra." Well, not quite. "Every" is a strong word and easy to quibble with.

The series will not include, for instance, the early Piano Concerto in E-flat, which was reconstructed from an existing piano part and reduced orchestral notations, or the better-known Choral Fantasia, a trial run for the Symphony No. 9, which has a significant role for solo piano. The much more dubious "Piano Concerto No. 6," the first movement of which has been completed by a modern musicologist, is not included, either.

But there will be something the NSO calls the Piano Concerto No. 6, a reworking of the great Violin Concerto in D for piano, rather than violin, soloist. Already it should be clear that when it comes to Beethoven's concertos, the usual names and numbers can be deceiving.

For instance: The "Second" Piano Concerto (being performed Sept. 18) was, in fact, conceived well before the first. It is numbered No. 2 only because it was published--set into some kind of permanent form--after the work known as the "First" Concerto. Which makes tonight's concert, featuring No. 1 and No. 5, not quite the bookends to Beethoven's piano concerto style that it at first appears to be.

In fact, there are no real bookends to Beethoven's concertos, which were in a state of continuing evolution not just as he wrote them but, apparently, as he played them. The hieroglyphs that frazzled Seyfried, and other contemporary accounts, suggest that Beethoven's performances of the first two piano concertos (apparently he performed the third and fourth only once, and the fifth not at all) may well have been very fluid, with much improvisation and spur-of-the-moment reconfiguring.

In some cases, such as the troubled Piano Concerto No. 2, Beethoven repeatedly redrafted the work for successive performances--leaving us the discarded Rondo in B-flat (which the NSO performs on Sept. 18), an early version of the last movement left on the cutting room floor.

The Beethoven scholar Leon Plantinga, in his magnificent new study ("Beethoven's Concertos," published by W.W. Norton), goes so far as to compare the works Beethoven frequently performed as a soloist to the oral tradition of Homer, in which each new performance yields a different work. At a distance of roughly two centuries, it's difficult to imagine what we now think of as great, finished masterworks as merely momentary forms taken by a now (mostly) dead performance tradition.

The complete concerto approach to this season's Beethoven Festival is based on this premise: that "complete" means playing all the finished, canonical works and anything else that directly relates to, or explains, them. This approach reflects the modern concert industry's focus on the accepted canon played by performers who struggle to give precise accounts of finished, authentic works with definitive texts that can be scrutinized and re-created.

Yet, more so than almost any form Beethoven used, the challenge of his concertos is twofold: to recapture the spirit of their first performance, the seat-of-the-pants tension that Seyfried must have felt while turning pages during the Third Concerto, while showing the form's evolution toward a more finished "great work"--like a symphony, grand, final and complete for posterity.

The best opportunity for audiences to hear hints of all this comes Sept. 17, when the orchestra performs the Piano Concerto No. 4 and the piano transcription of the Violin Concerto in D. The latter, a work whose solo part shows little trace of Beethoven (though it may well have been written out from his notations), is a ghastly thing, with few exceptions an almost amateurish transcription job. Rarely performed, it demonstrates one of the rare virtues of invoking the limits of a canon: to spare ourselves the abortive ideas and dead-ends that don't illuminate the great works.

The Piano Concerto No. 4, however, shows us where the very idea of "great works" begins to form. It was the last piano concerto Beethoven performed himself, at the premiere in 1808. Deafness was setting in, and the composer's stature was such that he was less financially dependent on playing the piano in public. Contemporary reports say Beethoven played "mischievously," taking liberties with his own music. Yet already in this concerto (such as it has come down to us on paper), one hears something new happening.

From the first strangely personal, self-absorbed solo piano entrance, to the last notes of the Concerto No. 5 ("Emperor"), Beethoven is altering the form, taking more dramatic chances on paper, leaving less to chance during the performance. He writes out the music definitively, and in the case of the "Emperor" Concerto he even notates the solo cadenzas (brief, often improvised passages for the soloist). In short, he begins to write these concertos for others, relying less and less on his own performance to complete the work.

As so often happens in Beethoven, one hears moments when the modern world begins to coalesce, with much lost and much gained. There is a gravity in the late concertos that presages the ills of today's world of traveling virtuosos, subscription concert series and Beethoven festivals. The late concertos overshadowed the earlier Beethoven concerto tradition, the spontaneous tug of war between a freewheeling soloist and an orchestra that can shout, but never improvise.

Yet there are elements of that earlier tradition that can redeem the existing canon and revive the drama of Beethoven's music, which is so often made inert in the very attempt to celebrate it. Some performers, such as the gifted pianist Robert Levin, who willingly improvises in classical style before the public, have started down this path. It's tempting to imagine a world in which it was the usual practice to improvise the early concertos, in which learning the notes was only the beginning for every pianist who approached these works.

The skill is not, in fact, so difficult. (Most period-instrument performers must be able to re-compose music on the spot on a regular basis.) The challenge would be primarily to our sensitivities, to a desire Beethoven helped create--the desire to hear music that is perfect, set in stone and preserved in the dead permanence of the "definitive" performance.

THE NSO'S BEETHOVEN BOUNTY

Here is the schedule for the 1999 Beethoven Festival at the Kennedy Center Concert Hall. All programs feature the National Symphony Orchestra:

* TODAY AT 7 P.M. Leonard Slatkin, conductor; Marissa Regni, violin; Andre Watts, piano. Romance No. 1 in G for Violin and Orchestra, Op. 40; Piano Concerto No. 1 in C, Op. 15; Piano Concerto No. 5 in E-flat, Op. 73 ("Emperor").

* SATURDAY AT 8:30 P.M. Leonard Slatkin, conductor/piano; Toshiko Kohno, flute; Daniel Matsukawa, bassoon; Jonathan Biss, piano; Hilary Hahn, violin. Romanze Cantabile in E Minor for Piano, Flute, Bassoon and Orchestra, revised and edited by Willy Hess; Piano Concerto No. 3 in C Minor, Op. 37; Violin Concerto in D, Op. 61.

* SEPT. 17 AT 8:30 P.M. Leonard Slatkin, conductor; William Steck, violin; Garrick Ohlsson, piano. Romance No. 2 in F for Violin and Orchestra, Op. 50; Piano Concerto No. 4 in G, Op. 58; Piano Concerto in D (Beethoven's own transcription of the Violin Concerto, Op. 61).

* SEPT. 18 AT 8:30 P.M. Leonard Slatkin, conductor; Kalichstein/ Laredo/Robinson Trio: Jaime Laredo, violin; Joseph Kalichstein, piano; Sharon Robinson, cello. Concerto Fragment in C for Violin and Orchestra, WoO 5, edited by Joseph Hellmesberger; Piano Concerto No. 2 in B-flat, Op. 19; Rondo in B-flat for Piano and Orchestra, WoO 6; Concerto in C for Violin, Cello and Piano, Op. 56 ("Triple").