Yan Pascal Tortelier has been named principal guest conductor, and Marek Janowski will serve in an endowed guest conductor chair.

The arrangement will be virtually unprecedented in the American orchestral world, where a music director generally serves as an ensemble's public face and fundraiser-in-chief, as well as overseeing programming and conducting a large number of concerts.

As artistic advisor, Davis will chair a "programming advisory committee" and conduct five weeks of concerts per year, starting in the 2005-06 season. He will maintain his post as music director of the Lyric Opera of Chicago.

Each of the conductors will focus on a segment of the repertoire. Tortelier, who is also conductor laureate of the BBC Philharmonic, will lead French, 20th-century, and 21st -century music. Janowski, the music director of the Berlin Radio Symphony, the Monte Carlo Philharmonic, and the Suisse Romande Orchestra, will specialize in German repertoire. And Davis will concentrate on British and American music, as well as leading a Dvoršk festival in 2006-07.

Perhaps the closest parallel to the PSO's new arrangement in the United States is the leadership of St. Paul Chamber Orchestra, which starting this season is conducted by four "artistic partners," currently including conductor Nicholas McGegan and violinist-conductor Joshua Bell. Other orchestras, notably the Atlanta Symphony, have presented their principal guest conductors as near-partners with the music director.

In a statement announcing the appointments, the PSO described its new arrangement as a positive and innovative step forward. Board chairman Richard Simmons said, "Ultimately, we chose a bold, new direction, one, which we feel, is the right answer for this point in our history and which we feel will serve our unique needs."

But in interviews with the New York Times and the Pittsburgh Post-Gazette, officials acknowledged that the unusual configuration was driven in part by a failure to find a single person to succeed Jansons, who stepped down earlier this year. Thomas Todd, the former board chairman who led the PSO's search committee, told the Times, "We did not feel that any one person had the capability to meet the job or to provide the time for the job at this time."