The first, and most immediately striking, is its extreme softness, a characteristic that earned Feldman a reputation as the "quiet man" of contemporary classical music; the painstakingly developed ppppp meniscus of a piece is more readily sustained within the pristine digital environment of CD than on crackly old vinyl. The second is the extreme length of his compositions, which in later works such as String Quartet No 2 and the four-CD For Philip Guston stretches to between four and five hours. It's now possible, for instance, to squeeze some of Feldman's medium-sized works on to a single CD, rather than have their subtle flow disturbed every 25 minutes or so, as on vinyl.
Feldman disagreed with those who regarded his pieces as too long. "In music, it's very difficult to distinguish between a thing's proportions and its form," he wrote. "My pieces aren't too long... they seem to fit the temporal landscape I provide. Would you say that The Odyssey is too long?" On another occasion, he described his Triadic Memories - a solo piano piece lasting between 70 and 90 minutes - as "the biggest butterfly in captivity", a perfect description of its enormous fragility.
For all its softness, this is truly monumental music: listening to it is like watching the surface of an immense boulder as the sun crosses the sky, observing the subtle alterations of light and shade as they navigate across the terrain. Long, slow, undulating passages, some involving just a handful of lonely tones per minute, are interspersed with the occasional enigmatic chord or flurry of notes, and at certain points, the same note may be played persistently over and over in quick succession, a technique that dissolves conventional notions of melody and narrative development, employing instead an aesthetic strategy more akin to the sculptural mobiles of Alexander Calder, with discrete tones held in equilibrium within a "listening plane". It's like a quieter, less disruptive development of Varese's drama-filled Musique Concrete, more concerned with arranging the sounds in a manner that establishes a precise mood or state of mind. Some commentators, indeed, have claimed that Feldman doesn't so much make music as "create consciousnesses".
Feldman had a great affinity with visual artists, and counted most of the famous Abstract Expressionists among his friends, writing pieces for the likes of Franz Kline, Philip Guston, Willem De Kooning and Mark Rothko. Their methods were a constant source of inspiration: Kline's statement that "I paint the white as well as the black, and the white is just as important", for instance, led Feldman to apply the same scrupulous attention to the balance between sound and silence in his own work, while a black-on-black canvas bought from Robert Rauschenberg for "whatever you have in your pocket" (just $17 and some change!) inspired the composer to emulate Rauschenberg's notion of dealing in "neither life nor art, but something in between". On another occasion, his understanding of the importance of scale was revised by watching Rothko have a large canvas re-stretched four times as he pondered whether to make it an inch or so smaller. From Rothko and Guston, too, came the appreciation of stillness which pervades Feldman's oeuvre. "I'm involved in stasis," he once said. "It's frozen, at the same time it's vibrating." This paradoxical state is perhaps best evoked in the numerous pieces written for glassy timbres such as piano, glockenspiel, string harmonics and tubular bells, in which the high-register tones dissolve ambiguously into a single glowing whole, frozen and vibrating.
In later years, Feldman became fascinated by antique Anatolian carpets, an interest that enabled him to die rich after struggling for much of his life (until the age of 44, when he was offered the post of Edgard Varese professor of music at the University of New York at Buffalo, he worked for his uncle's dry-cleaning business). The carpet designs led him to a deeper understanding of pattern and proportion, and the way that repetition of detail dissolves strict notions of symmetry and asymmetry in large-scale works.
For all its deceptive tranquillity, listening to Feldman's music requires a certain dedication on the part of the listener. Playing it, of course, involves another level of attention entirely. "It's like running a marathon," says the pianist John Tilbury, whose four-CD set of Feldman's complete piano works is released in a few weeks' time. "You know exactly what's expected, it's not a surprise, and you've trained for it. But it's not just the length; it's the softness, too, and the precision that's required can be quite tiring.
"Playing Feldman requires great concentration; playing a sound, listening to it decay, and then making decisions about how to play the next one; finding the balance within chords, and thinking about the weight of chords. It's a bit like the painters Feldman admired, putting paint on, then standing back to watch it dribble across the canvas. The music has a kind of autonomy; it does its own thing, depending on the instrument - on a Bosendorfer, say, the low bass notes seem endless, with a quality and depth you don't get on other instruments."
"I think clarity of utterance is the key in playing Feldman, you need great fingertip sensitivity and control. That's where the paradox comes in - you need this tremendous fingertip sensitivity to control the chord, then you sit and listen to the sound decay, completely out of your control, before playing the next chord in as controlled a manner as possible. So there's this constant dichotomy between control and accident. That's one of the most interesting facets for the player."
It's a dichotomy that reflects the dualistic nature of Feldman's music, which is both intuitive - in the sense of recognising which sound, in the absence of formal support structures such as harmony, sits correctly within the flow of a piece - and highly nuanced, with the finest possible gradations of tone, time and inflection painstakingly determined. Compared with the rude subversions of most avant-garde music, listening to it is an enticing, relaxing experience that gently imposes the mood of the music upon the listener.
"It invites the audience into a world of sound," agrees Tilbury. "There's also a strong impression of the creation of space, rather than of filling space, which is what usually happens not only in music, but in general: television screens have to be filled with images, streets have to be filled with cars, manuscript paper has to be filled with notes. But Feldman seems to be able to create space: the music breathes, and it allows the listener to breathe as well.
"It encourages a state of being intensely aware, too. People talk about its meditative qualities, but when I play it, I feel incredibly alive and aware and involved - and aware of environmental sounds, too. I remember playing in Austria and being aware of some wonderful church bells that were floating through the window and providing a miraculous counterpoint to the music. It's a very intense state of being that's the crucial characteristic of this music - it's good to be alive and here, listening now. It's the `nowness' of the music, I think."
John Tilbury's recording of Feldman's complete piano works is released on London Hall Records this month