At first, the Mendelssohn Choir was really the Men-delssohn Choir.

Only tenors, baritones and basses existed in its first permutation: the Mendelssohn Male Choir. In September 1908, conductor Ernest Lunt gathered approximately 20 men on the second floor of the Hamilton Music Store on Wood Street near Liberty Avenue for a rehearsal of a new chorus. A few months later the numbers had increased to 40. And in January 1909 -- the 100th anniversary of the birth of its namesake, Felix Mendelssohn -- the men debuted at Carnegie Music Hall in Oakland.

That single-gender situation wouldn't last long. In 1914 a Mendelssohn Ladies Choir formed, and the next season the men and women combined to form the Mendelssohn Choir of Pittsburgh. The mixed chorus has been a stalwart of the city's music scene ever since, giving concerts on its own, touring and joining the Pittsburgh Symphony Orchestra for major choral works. The choir is the oldest continuously running arts organization in Pittsburgh, a major accomplishment in a city with many venerable institutions. On Friday, it will celebrate 100 years of music-making with a concert by a work of, who else, Mendelssohn: his oratorio "Elijah."

"It takes an awful lot of work to make the Mendelssohn what it is -- a body of talented singers and enormous behind-the-scenes efforts," says current music director Betsy Burleigh, who has headed it since 2005. "For 100 years they have made it work."

"This is significant milestone," says Ann Meier Baker, president of Chorus America. Although that service organization estimates there are 250,000 choruses in the United States, it can list only a handful of active choirs older than the Mendelssohn Choir of Pittsburgh. Among these are the Bach Choir of Bethlehem (founded in 1898), the Singers' Club of Cleveland (1891), the Mendelssohn Club of Philadelphia (1874), the May Festival Chorus (1873) and the Handel and Haydn Society (1815).

"If you are 100 years old, clearly you have weathered a number of transitions in leadership, economics and public taste," says Meier Baker. "[The Mendelssohn Choir has] strong muscles and stick-togetherness. It has always had a significant profile."

The credit for the stability goes in large part to having the same director for the first 42 years of its existence. The British-born Lunt was known for his perfectionism, and it was during his tenure that Pittsburgh first heard such works as Brahms' "Requiem" and Bach's "B Minor Mass." When the Pittsburgh Orchestra, as the PSO was known at the time, went out of business (from 1910 to 1926), the enterprising Lunt collaborated with the Philadelphia Orchestra, the Cleveland Symphony Orchestra, the New York Symphony Orchestra and others. And in 1919, with women on board, he took over the yearly tradition of presenting Handel's "Messiah."

In 1929, the choir's relationship with the "new" PSO began in a season that included choral works by such composers as Elgar, Brahms, Holst and Beethoven. The group continued to grow in numbers and prestige under Lunt. But his retirement in 1950 placed a serious challenge in front of the membership.

"They were wondering if they should continue the choir," says Jeanne Miller, who joined that year as an alto.

"It is not unusual for a chorus to be so connected to its conductor that it can falter when the conductor moves on or dies," says Meier Baker. "It is a strong bond between the two. It would have been easy for the choir to disband or for choristers to move on to some of the wonderful other choirs in the Pittsburgh community."

That didn't happen. In fact, the choir went in the opposite direction, thinking big.

"They took the bold step of inviting Robert Shaw to be the guest conductor of that season," says Miller. The iconic choral conductor programmed Bach's "St. John's Passion" and Mozart's "Requiem," as well as establishing a formal connection to sing "Messiah" with the PSO. "It [set] the choir back on its path."

Shaw's stint gave the group momentum it has never relinquished under later directors Russell Wichmann, Henry Mazer and Hugh Johnson. There were performances at the Chautauqua Institution and with the Pittsburgh Ballet Theatre, plus several high-profile concerts, such as a memorial concert for President John F. Kennedy. And with PSO music director William Steinberg, the choir and the orchestra started performing more works together on subscription concerts.

The period also marked the entrance of two shining examples of the essential "behind-the-scenes" work of which Burleigh spoke earlier. In the late '60s, Jeanne and her husband, Barry Miller, a bass in the choir, began to serve as the administrative backbone of the group -- Barry as operations manager and Jeanne as membership secretary. The chorus's longevity also is due to the stability they have provided for the past 40 years.

"They have been the heart and soul of the Mendelssohn for so long," says Burleigh.

In 1979, renowned conductor Robert Page took the helm, bringing a new level of singing by instituting a professional core to the group. These singers, usually numbering 20, get modest pay ($14 an hour today) for rehearsal and performances. All the singers pay an annual membership fee of $160.

"It has inspired our volunteers to take lessons, to improve their sight reading and to do more homework," says Barry Miller. "They set a good example, and the quality of the choir has improved with the inception of the core singers."

The choir became much more involved with the PSO, with the orchestra even naming Page its director of special projects and choral activities, while continuing to branch out more on its own.

"Bob's goal was to do major works that the symphony would not perform," says Barry Miller. "Works such as Britten's 'War Requiem' and Shostakovich's 'Babi Yar' Symphony, and smaller choral works." In 1986, Page and Christine Jordanoff formed the Junior Mendelssohn, bridging the gap between the Children's Festival Chorus and the adults.

Page retired in 2005, after 26 years at the helm. He is still active with the group as an emeritus music director and will conduct "Elijah" on Friday. Under Burleigh, the group now numbers about 115 singers.

Financially, the Mendelssohn Choir is on solid ground, having balanced its $300,000 budget last season, but it hopes to increase its endowment. "The endowment started decades ago, but six years ago the Mendelssohn had to go into the endowment to make ends meet," says Barry Miller. "We know the amount that we have to replenish."

The choir is looking to do this more through fund raising because between 50 percent to 60 percent of its income comes from its contracted work for the PSO, work that is inconsistent and based on the programming of the orchestra.

"We treasure our partnership with the PSO," says Mendelssohn board president George Seeley. "Being a self-standing, independent organization, we are well aware that engagements with the PSO can wax and wane. They do not pay enough to maintain the organization; we have to do fund raising."

That's a slow process, with disappointments and successes, but Seeley and the board are committed to the plan. "In order to keep the singers at the level you need, you have to keep the quality going. You can't rustle up people in a short time period." The group soon will hire a new executive director to help keep it on course.

For her part, Burleigh wants to increase the number of premieres the group gives and the national profile of the group. Both will happen at a special Library of Congress concert celebrating Felix Mendelssohn in Washington, D.C., next February.

"In the coming years," she says, "I hope we can show the world what we have in Pittsburgh."