For composers, performers, and audiences alike there is something inescapably special about the string quartet. In repertoire and reputation it has been the consummate chamber music medium since Joseph Haydn made it so in the 18th century. A medium for connoisseurs, enthusiasts, and fanatics, this most flexible and voice-like collection of instruments seems to be the most intimate utterance of its composers. To create a quartet is a fraught endeavor; you are measuring yourself against supreme masterpieces. Mozart absorbed Haydn's "Sun" Quartets and girded himself for his own carefully crafted collection, which he dedicated to Haydn. Beethoven spent years grappling with the legacy of Haydn, finally producing a group of quartets in his mature voice that have inspired and haunted composers ever since. This situation is the foundation for the Emerson String Quartet's upcoming Perspectives concerts.

Emerson violinist Eugene Drucker says that the essential concept of the concerts is Beethoven, the composers who influenced Beethoven, and the composers he influenced — which is to say, nearly everyone who followed him, some taking his lead, some rebelling. Like the Emerson, most ambitious ensembles eventually mount a series of all 16 Beethovens, and generally at the same time learn the "New Testament": the six quartets of Bartók that loomed over the later 20th century. In recent years, the Emerson Quartet has revealed the full power of the other great set of the century, the 15 by Shostakovich. But Beethoven remains the giant star around whom other composers orbit, with greater or lesser independence.

A Beethoven quartet series is often more or less chronological, as is this one: from the backward-looking early Beethovens to the radical and sublime late quartets; from Haydn and Mozart through Schubert, Mendelssohn, and Brahms; from Bartók, Webern, and Ives to Shostakovich, Rihm, and Saariaho. For seasoning, there is a visit to Bach's Art of the Fugue, another synoptic collection of incomparable scope, reminding us that Beethoven spent his youth playing Bach on piano and his last years composing fugues in the spirit of Bach.

The Emerson's first program presents three quartets interpreting C major — the key, Drucker says, which, with its lack of sharps or flats, can be called the ultimate Enlightenment tonality: the mean, the middle ground. The Haydn "Bird" quartet is one of the "Sun" group that inspired Mozart's "Haydn" set, whose most famous member is the "Dissonant," named for its harmonically wandering introduction. Beethoven's Op. 59, No. 3 is from his group of three "Razumovsky" quartets, in which after his more cautious Op. 18, composed in Haydn's shadow, he remade the medium with three revolutionary works — yet at the same time, the unsettling introduction to No. 3 harks back to Mozart's "Dissonant."

The next evening is a double concert with dinner break between, presenting the six quartets of Beethoven's Op. 18. Drucker notes that here the composer is "poised on the border between Classic and Romantic," which is to say, between the past of the string quartet and its future. In Op. 18 he is at once looking back to Haydn and experimenting, notably in No. 6, whose course comes down to a movement called "La Malinconia," melancholy; that melancholy returns to haunt the finale, just as later the scherzo will return in the finale of the Fifth Symphony.

The next concert evokes the genius and maturity of the 18-year-old Mendelssohn, in Op. 13 already showing his assimilation of the late Beethoven quartets. Beethoven's "Razumovsky" No. 2 follows, then Schubert's "Death and the Maiden" Quartet, integrating his eponymous song, written in the shadow of his own death.

The following program joins Beethoven's "Razumovsky" No. 2 with the first Brahms quartet (rather, the first he published after 20 attempts in a medium he dreaded) and Ives's String Quartet No. 2, a piece that is at once Beethovenian and anti-Beethovenian — it pokes fun at the Ninth Symphony but winds up on a mountaintop in a high-Beethoven stretch of sublimity.

Then three 20th-century masterpieces by Webern and Bartók are placed alongside two contrasting Beethovens: the warm and expansive "Harp" quartet and the searing, succinct "Serioso." Drucker calls this program a study in composers' impulse to expansion and the contrasting quest for conciseness. The latter quality is epitomized in Webern's extraordinary Five Pieces, miniatures that speak volumes.

A study in introductions unifies the next program, two of the uncanny last quartets of Beethoven joined by one from the prolific, contemporary atonalist Wolfgang Rihm. Here is the distillation of what Drucker calls Beethoven's "pro-structuralist but anti-formalist" tendency: he pursued an ultimate integration of all traditional elements, theme to theme, movement to movement; and as part of that transcending of boundaries, in the Op. 127 and 132 Beethoven rethought the function and meaning of introductions. Rihm, meanwhile, introduces his quartet with a curt presentation of his central motive.

The penultimate program joins excerpts from Bach's Art of the Fugue with the devastated and devastating last Shostakovich quartet (its first movement fugal) and Beethoven's most radical expansion of traditional form, Op. 130, along with the wild contrapuntal study, the Grosse Fuge — a work of endless fascination and freshness, eternally avant-garde.

For the final program, the last two Beethoven quartets and their two different worlds are presented side by side: the puckish and retrospective Op. 135, and the Op. 131 in C-sharp minor, tragic even in its key, whose taut and severe fugal opening Wagner described as the saddest music ever written. In between is the work completing the circle of the Emerson's Perspectives, the world premiere of Terra memoria by Kaija Saariaho, written in memory of those — in all of our lives — who are lost to us.

A set of programs like this one is a journey. And in the medium of the string quartet, Beethoven's is the great and definitive journey, his music encompassing the past and engendering the future. As these Perspectives show, that journey is still vitally in progress.

Composer and author Jan Swafford has written biographies of Charles Ives and Johannes Brahms.