Pittsburgh Symphony Orchestra principal horn player William Caballero has the French horn equivalent of a triple threat. "This is an unusual horn, instead of a single or a double horn, this is a triple horn," he says. "It actually has three horns built in together and works quite well for the needs of a principal horn or high horn player."

It's not that Caballero isn't capable of playing even fiendishly high and difficult parts on the traditional horn -- or even the older, natural horn without any keys -- but the triple technology gives him peace of mind as he performs the grueling PSO schedule.

"I have a horn I like a lot," he says. "I can play in the morning and put it away, and there is a big difference from worrying about a concert to enjoying it. That translates to our section. You can tell if we are enjoying the concert."

Most of the time when you hear a solo from Caballero, who joined the PSO 21 seasons ago in 1989, it's within symphonies. Composers love using the French horn to add its mellow and noble sound to orchestra works. But they didn't seem to love it enough to feature it as a solo instrument in its own concerto.

"There are not that many, and a good portion [of those] are not very good," says Caballero. "We have the war­horses: the four Mozarts plus fragments, the two Strauss concertos, Haydn's and the Gliere." But even these don't get performed much. "Mozart Four and Strauss One have not been played by the PSO," he says.

So horn players around the country are excited about the potential addition to the repertoire of a new concerto by John Williams, who wrote it in 2003 for Chicago Symphony Orchestra horn player Dale Clevenger. While Williams has used the French horn to great effect in his famous film music, this concerto often doesn't even sound like the same composer.

"Right off the bat you are going to hear a little bit of John Williams and then it is gone," says Caballero.

"You would never know this was a film composer or even Williams," says Leonard Slatkin, who will conduct the PSO and Caballero in the work this weekend at Heinz Hall.

"When I've tried to analyze my lifelong love of the French horn, I've had to conclude that it's mainly because of the horn's capacity to stir memories of antiquity," writes Williams, who has now composed several concertos, including for violin, cello, clarinet, flute, bassoon and tuba. "The very sound of the French horn conjures images stored in the collective psyche. It's an instrument that invites us to 'dream backward to the ancient time.' "

Indeed, Williams' score for the piece comes with a medieval woodcut of a hunter blowing an animal horn, and eachh of the five movements of the concerto comes with a title and a poetic description:

I. Angelus. "Far far away, like bells at evening pealing"

II. The Battle of the Trees. "Swift Oak ... Stout Guardian of the Door"

III. Pastorale. "There Came a Day at Summer's Full"

IV. The Hunt. "The Hart Loves the Highwood"

V. Nocturne. "The Crimson Day Withdraws"

"Most cultures have had some form of horn in their histories," writes Williams further. "We remember the ram's horn Shofar, calling us to battle or prayer. Or the conch, 'fabled shell instrument of the Titans,' or one can imagine the huge Viking horns that must have struck terror in the hamlets of northern Europe as the great ships were brought into the estuaries to begin their attacks. The horn stirs memories of fearful things, of powerful things, of noble and beautiful things!"

Williams might admire the horn, but that doesn't mean he makes it easy on the horn player.

"It is a long piece for a horn concerto," says Caballero, who grew up in Kenosha, Wis. "[It has] an expanded range; there is a lot of low as well as high playing." There is one particularly "very challenging spot" in which he is asked to play an extended passage with the hand in the bell of the horn to alter pitch.

"It is hard," says Slatkin. "The virtuoso movement, which would be the last, is earlier."

But for Caballero, the last movement, the lyrical and poetic Nocturne, is the gem of the work. "He captured the horn in a completely surprising way in the Nocturne."

Placing this quiet movement at the end, instead of in the middle of the piece, lends an extra tinge of sadness and weariness to the finale simply because the lips of the performer are wearing out by then. "Williams was not sure about [the placement], and by the time you get to the last movement your lips are tired and you have this lyric movement to put together," says Slatkin, who recorded the work with Karl Pituch and the Detroit Symphony Orchestra (to be released on Naxos). But that seems to lend an extra ambiance of nostalgia to the movement. "It just fades away," says Slatkin.

Horn players everywhere are hoping this new concerto does not follow suit.

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