On Sunday, Musicians from Marlboro perform a concert at the Isabella Stewart Gardner Museum including Haydn's String Quartet (Op. 76, No. 5). The work is in D major, but its slow movement (source of its sometime nickname, the “Largo”) is in F-sharp major: six sharps, nearly every note inflected — in tonal music theory, as far away from C major as one can get. A rare choice then; even now, in a largely equal-tempered, chromatic musical world, it retains a whiff of its original, extreme nature.
Practically nonexistent before the 1700s, F-sharp major became an option as new tuning techniques expanded chromatic possibilities. Older tuning systems based on chains of pure intervals rang true in keys near the tuning center (C, usually) but discordant if the harmonies modulated too far afield. Tempering the scale — massaging intervals to more closely approximate pure ratios in multiple keys — produced “well-tempered” systems, famously inspiring J. S. Bach's two books of preludes and fugues. But well-tempered was not equal-tempered: The most common system, invented by Andreas Werckmeister, still left the distance between F-sharp and A-sharp, the core of the tonic triad, noticeably wide. (Bach correspondingly downplayed that interval in his F-sharp major preludes and fugues; Book I's fugue subject, for example, avoids it.)
The key's novelty appealed to the ever-experimental Haydn, who first used it in one of his most famous works: his Symphony No. 45, the “Farewell” symphony, written to persuade his patron, Prince Nikolaus Esterházy, to release his musicians from palace service to return to their families. Part of the argument involved shifting the symphony's prevailing F-sharp minor tonality to major: a luminous, distant vision of home and reunion.
Well-tempered tuning systems soon evolved into equal temperament, all keys sharing the same ratios. By 1809, Beethoven could write an entire piano sonata in F-sharp major (Op. 78, one of his favorites). But as European music turned more chromatic, F-sharp major once again came to mark a kind of threshold. Schoenberg's Second String Quartet, one of his last way stations on the journey from expressionism to full atonality, ends its final, Odyssean movement in F-sharp major. It was the key of Mahler's unfinished Tenth Symphony. It was a key that Messiaen, the most mystical of 20th-century composers, reserved for maximally transcendent moments. Brought into the fold, acoustically and mathematically, F-sharp major could still symbolically mark the frontier.
Musicians from Marlboro play Jan. 22 at the Gardner Museum. 617-278-5156, www.gardnermuseum.orgMatthew Guerrieri can be reached at firstname.lastname@example.org.