Beethoven wrote, but never delivered, a love letter to his Unsterbliche Geliebte ("Immortal Beloved"). The matter of her identity is far from settled, with scholars each passionately defending their own candidates. Whole books have been written on the subject. Antonie Brentano, wife of Franz Brentano, a well-to-do Frankfurt merchant, is one of the candidates. Whether Antonie was Beethoven's beloved or not, the Brentano String Quartet is certainly the geliebte of music lovers worldwide.

The quartet will play a concert on Tuesday at Bass Performance Hall in Fort Worth as part of the Cliburn at the Bass Concert Series. Most importantly for local audiences, they will also be the string quartet for the chamber music section of the upcoming Van Cliburn International Piano Competition that runs May 24-June 9. It is their first time to be part of the competition. 

On Tuesday, they will play a greatly varied program that ranges from music by the British Baroque composer, Henry Purcell, to Belá Bartók's intense and prickly String Quartet No. 4. Haydn's String Quartet No. 35 and Brahms' String Quartet No. 2 complete the program. 

The Brentano Quartet is currently in residence at Princeton University, a famous enclave of contemporary composers. Their recording of Beethoven's Opus 131 is also used in the new film A Late Quartet, and the group's cellist, Nina Lee, has a cameo as herself in the film.

"We were interested in working with composers right from the start in 1992," says first violinist Mark Steinberg. "You can ask them questions and interact about the music, which is something you can't do with the older composers." 

Considering how active the Brentano Quartet is with living composers, it is a shame that one of their commissioned works is not on the program for Tuesday. 

Steinberg is energetic and personable with a breezy way of making serious points about music and performance. One interesting fact is that, like the two top violinists in the Fort Worth Symphony, both he and second violinist Serena Canin play on Stradivari violins that are on loan. Violist Misha Amory and cellist Nina Lee also play on fine instruments, although they do not have a set of four matched Strads, such as the instruments currently on loan to the Tokyo String Quartet. 

"These are two of his later violins. They are a little darker as opposed to the earlier ones which were a brighter. However, they offer a greater variation of color," he says. "While a good player can make any acceptable violin speak, you can get more information into the music with a great instrument." 

Strads are famous for being moody and not as ready to respond as some of the other great violins, such as the instruments by Guarneri del Gesù. "You  have to find your way into the instrument and find how to make it respond," Steinburg adds. 

You often wonder what players think as they are just walking around with a multimillion dollar, irreplaceable and historic instrument in a case in your hand. 

"That is always in the back of your mind," Steinberg says. "You can't think about it in the foreground as you are walking down the street or you would go crazy and never go anywhere. But, when we sit down to play, the value of the instrument is front and center. It is always a wonder to hear these two instruments together," he says. 

There are stories about players leaving them on busses or in a taxi. Worse, slipping on the ice one winter's day and driving the sound post through the back of the instrument, which would make it valuable for little more than kindling. 

Sound posts are a small piece of wooden dowel that is precisely wedged between the front and back of the modern string instruments just under the highest string on the bridge.  The exact position is critical and it must be adjusted by an expert. A movement by the smallest fraction of an inch can completely change the sound. 

"Some players have their instrument in the shop all the time having adjustments; some don't do it for years," Steinberg says. "I tend more to the side of less. I am most likely to have the sound post adjusted when the weather changes. This is because the instrument is dryer in winter and wetter in the summer, so the wood changes." 

On Tuesday, with winter wood just barely in place, the quartet will give us a preview of what to expect when the pressure is really on in May as the contestants for piano's most coveted prize each take a turn to make music with the Brentano Quartet for the judges.