Elizabeth Schnobrick is the National Symphony Orchestra’s principal librarian. She prepares the music for a single subscription program. (Jonathan Thorpe for The Washington Post)

When an orchestra takes the stage, all eyes, and ears, are on the music. But before the musicians can play, someone has to make sure they have the sheet music to play from. The orchestra’s librarians are responsible for obtaining the scores for each program; for making sure all of the markings are the same in all the parts; for putting the music on the stands before each performance; and for being physically present at each show in case of emergencies — even on tour. It takes about 100 hours, says Elizabeth Schnobrick, the National Symphony Orchestra’s principal librarian, to prepare the music for a single subscription program.

The orchestra owns hundreds of parts, housed in ranks of dark file cabinets in a narrow room off the main library in the NSO’s administrative offices at the Kennedy Center. The music is sorted not alphabetically, but according to when the orchestra played it — starting, appropriately, with Beethoven’s First Symphony. (Sorting chronologically means the librarians don’t have to rearrange all the music if, for instance, the orchestra acquires more scores by a composer beginning with B.) As soon as the season is announced — this year, in January — the librarians (two full-time staffers, one part-timer) set to work.

“The first thing we do is take the program that the artistic team puts together with our conductors, and we do a lot of research,” says Schnobrick. “The very first thing is sourcing. What do we own, what don’t we own. If we don’t own it, can we buy it?” Works under copyright must be rented from the music publisher, and a fee must be paid for each performance, rather than a flat fee for using the parts. “Is the conductor sending it?” Some conductors keep libraries of parts with their own preferred markings. This season, Sir Mark Elder is sending three sets of parts from his personal library to the orchestra for his two-week residency in May.

The questions go on. “Are there multiple editions [of a score]? Is it a simple matter of different printings, or are there substantive differences” — that is, different versions of the same piece, like a Bruckner symphony, which the composer himself frequently revised? And then there are practical considerations. “If we own a piece of music, is the paper strong enough to last through another subscription cycle, or do we have to replace it?” Schnobrick continues. “It’s just a gigantic checklist.”

Schnobrick and her two colleagues, Susan Stokdyk and Nicholas Greer, submit their rental requests to the music publishers by June so that there’s plenty of time to get the parts by fall. Once the music comes in, they go through it painstakingly. “Every single piece of paper that goes onstage, every page is looked at by the librarians before every program,” Schnobrick says. Because musicians need to have their parts a month before a given program, to have time to practice, the librarians start preparing them about two months before any given performance — except when they can’t. The music for Thursday’s pop concert with Frankie Moreno is still rolling in; the last pieces are scheduled to arrive on Monday.

There’s no formal training program or degree for orchestra librarians. Schnobrick was a music educator who taught band and orchestra in a school for seven years before taking a year off, getting an internship with the librarian of the Philadelphia Orchestra and liking the work so much that she never returned to teaching. “The MLS degree doesn’t really apply,” she says, referring to the master of library science, “because we’re editing.” That is, actually making changes to the texts, rather than simply preserving them.

The librarians are not part of the administration, but an official part of the orchestra’s roster. “We are musicians,” says Schobrick. “We’re here to support performances.”