[Editor's Note: "Overtones" is a weekly opinion column by Washington Post classical-music critic emeritus Joe McLellan.]

Lovers of classical music tend to have their eyes fixed, firmly and fondly, on the past.

The current celebration of the 300th anniversary of the piano is an example, as are the many Mozart festivals that tend to pop up all over the map at this time of year, the worldwide observances of the 250th anniversary of Johann Sebastian Bach's death, and the week-in, week-out programming of just about any symphony orchestra or opera company you can name.

In terms of recognition, respect and frequency of performance, a living composer today has to compete with Ludwig van Beethoven in a way that novelists do not have to compete with Charles Dickens or Henry Fielding.

This was not always the case.

Except for music used in religious ceremonies, there was relatively little interest in old music until the 18th century. Then, in London, in the lifetime of George Frideric Handel, an organization called the Academy of Ancient Music was formed to cater to a specialized taste for music of the past.

It was considered a curiosity — quite antiquarian — because it revived and performed pieces that were as much as 20 or 30 years old.

In the next generation, after the transition from baroque to classical style, Bach and Handel became "ancient music." Mozart was hired by Baron Von Swieten, an official of the imperial court in Vienna, Austria, to modernize the orchestration of Handel's Messiah with clarinets and other state-of-the-art instruments.

Today, what we call "early music" is one of the most flourishing branches of the classical-music industry, with concerts, recording, publishing, festivals, fan magazines and instrument-making at an all-time high. It is second only to opera in its appeal to young audiences. And in attracting crossover enthusiasts who never go to a symphony concert, it is second to nothing else in classical music.

In spite of this vigorous interest — or perhaps because of it — there is no universally accepted definition of exactly what "early music" is.

What Makes Music 'Early'?

There are essentially two ways to define it: by the date of composition or by the kind of instruments being used.

In terms of date, for a long time "early music" was generally accepted (if not formally defined) as music from roughly 1750 or earlier to about the end of the Baroque era.

That date worked fairly well in terms of instruments, too. It was the generation when the viola da gamba lost its popularity in favor of the cello. The piano was in its infancy at 50 years of age, and the harpsichord was still holding its own, but its days were numbered.

The ancient chalumeau, a wind instrument popular in the Middle Ages and Renaissance, had been transformed into the bottom range of the clarinet, which Mozart would raise to the highest level of classical acceptance.

Old Instruments New Again

Today, the lute, harpsichord, viola da gamba and chalumeau, together with other long-abandoned instruments, have come energetically back to life. So far, the legendary art of the castrati has not been fully revived, but there are countless countertenors singing in the alto range, whereas 50 years ago there were only two.

The return to period instruments in early music has not been limited to reviving medieval, Renaissance and baroque instruments. It has gone on to restore the violin, piano, oboe and other modern instruments to the forms they had in earlier times.

The Boesendorfer concert grand on which Alfred Brendel plays Mozart is enormously different from its ancestor, the fortepiano on which Malcolm Bilson plays Mozart.

The Stradivarius, Guarneri or Amati violins used by such modern virtuosi as Gil Shaham or Anne-Sophie Mutter have undergone significant changes since they first left the hands of their makers some three centuries ago. They now have metal strings that sound louder and more brilliant, and their structure has been reinforced to sustain the higher tension of modern concert pitch.

Performances on period instruments have been advancing through music history, taking on music of Haydn, Beethoven, Chopin up to Brahms, whose Hungarian Dances have a different flavor when they are played on a violin with gut strings and tuned to a lower pitch, and avoid the continuous vibrato that violinists use today.

I have even heard musicologists suggest that a performance of that monument of modernism, Igor Stravinsky's Rite of Spring, would be more "authentic" if it used 1913-vintage instruments with the rather different style of playing heard at that time.

Does this mean that the use of period instruments has reached the 20th century?

It does, although this is still an advanced position and not as firmly occupied as those keyed to earlier centuries. Can we look forward to a time when, as in baroque London, an Academy of Ancient Music will be playing works from the 1970s and '80s?

Perhaps. I keep remembering a remark by John Harbison, one of America's finest living composers, at a discussion of period instruments a few years ago: "A century from now, I don't want anyone to be considered an expert on my music because he happens to play a 1970 Steinway."