The new year has brought with it an addition to the Philharmonic's program notes: an estimated duration for each work in the performance. Determining this number, however, is more of an art than a science.

A work's length results from the conductor's‹sometimes the soloist's ‹ interpretive choices. They set the basic tempo, of course, and also make many other decisions that affect a work's duration. How they answer questions such as the meaning of allegro (how fast is fast?); how much to slow down as a movement ends; whether to follow the composer's suggested repeat of a passage: therein lies music-making. Often the same conductor answers each question slightly differently on adjacent concerts. It is just this unpredictability that electrifies live performances.

The person at the Philharmonic responsible for quantifying the unquantifiable is Principal Librarian Lawrence Tarlow. (His estimates represent only the music‹the pauses between movements are not included.) For world premieres, composers provide timings. For other pieces new to the Orchestra, guidelines are found in reference books and publishers' catalogues. When there is a Philharmonic performance history, Mr. Tarlow's source is the Orchestra's own records, which can go back as far as World War I; to this he adds his knowledge of each conductor's style. "You can say that one conductor likes expansive tempi," Mr. Tarlow explains. "Or if a conductor supplies his own music, it will be clear if he takes the repeat . . . unless he changes his mind."

Looking at past timings is not only a guide to the future; it can be a window on the past. Take Schumann's Manfred Overture, performed just last month. A yellowed inventory attached to the orchestral parts' container (left) provides a default timing of 11 minutes, along with handwritten names of luminary conductors and their actual durations, among them "Walter" (10'50"), "Schippers" (12'40"), and "Leinsdorf" (12'30"). Then: "11'. Nov. 14/43. L. Bernstein on account of B. Walter illness." This last entry will resonate with many music lovers, as it was the concert in which Mr. Bernstein, then the Orchestra's young Assistant Conductor, stepped in at the last minute to replace the ailing Music Director on a nationally broadcast concert, thus launching his legendary career.

Monica Parks is a Publications Editor at the New York Philharmonic.