Certainly a leading candidate for most ungrateful wretch in the history of music -- including even the shameless Richard Wagner -- is Prince Ludwig, Margrave of Brandenburg.

He met Johann Sebastian Bach in 1719 when the composer was visiting Berlin to buy a new harpsichord. Bach's reputation had preceded him and the Margrave asked him to write something nice for him.

Bach took two years working on the music.

Then a set of six concertos arrived on March 24, 1721, with a groveling note of apology.

Perhaps the margrave was too busy at the time. For he never even acknowledged receipt, never sent a fee and apparently left the works on a shelf somewhere until long after the composer's death 29 years later.

At least the margrave can be credited with not destroying them. Otherwise, we would be missing the Brandenburg concertos -- the most celebrated orchestral works of the entire Baroque period and among the most popular works in the history of music.

The Brandenburgs were neglected back then. But three of them certainly get their due in tonight's "Great Performances" broadcast (Channel 26 at 9 p.m., with simulcast on WETA-FM 91) in versions by that exemplary Viennese early instruments ensemble, the Concentus Musicus under Nikolaus Harnoncourt.

Here is all the unquenchable vitality and variety that Bach poured into these works. He seemed set on redefining the range of the concerto -- all six feature different solo instruments -- on a scale that no composer had ever attempted, and that would not be matched until more than half a century later when Mozart came along.

As the announcer points out, the most "forward looking" is the Fifth (which concludes the broadcast) with its three soloists -- transverse flute, violin and -- by far the most important -- harpsichord. In this work, Bach in one fell swoop created the modern keyboard concerto. The colossal keyboard cadenza at the end of the opening movement -- a creation in which Bach keeps twisting the music to extraordinary levels of tension -- is the forerunner of the greatest cadenzas of Mozart, Beethoven and Brahms, and is as fine as any of them. It is played with splendid discipline here by Herbert Tachzi.

There are also fine performances of the Fourth and the Second -- appropriately enough for public broadcasting in the latter, for parts of this brilliant trumpet work provide theme music for both "Great Performances" and "Firing Line."