The longer one lives with Schubert, the more moving and, paradoxically, the more unfathomable he becomes. It is his lyricism—sublime, simple, seemingly effortless—that first captures the ear and the heart. Then one might discover Winterreise and, with it, the horror that complements and complicates that lyrical impulse. But then you hear the late instrumental music—the C-Major Symphony or the E-flat–Major Piano Trio—in which those same qualities are applied to a massive canvas. These pieces are a revelation. Schubert’s music is not merely beautiful, profound, and confronting: It is grand. Its scale is epic; its vision is staggering.

And then comes another revelation. Of the nearly 1,000 works Schubert wrote in his solitary, impoverished, syphilis-plagued, 31-year lifetime, it was not just the relative few that made the hit parade that demonstrate his magnificent, inimitable gift. No. Open your ears further, and you will find that one little-known piece after another is a world unto itself, each suffused with such tragedy and such tenderness, each offering a window into a different corner of Schubert’s soul.

This is the space that Schubert’s music for piano four-hands occupies. Apart from the Fantasie in F Minor—a decidedly un-celebratory work trotted out for many celebratory concerts—most of it is unknown to audiences. Apart from a few dedicated piano duos, much of it is unknown to pianists. What a shame! It would be difficult to overstate the richness of this music, or the extent to which it rewards an immersion in it. In true Schubert fashion, it can be massive in scope, but it can also evoke feelings of overwhelming intensity through a lone harmonic shift, or turn of phrase, or sleight of counterpoint so unassuming, any attempt to explain the source of its power would be an exercise in futility.

Each of the four works on this program is riveting for a different set of reasons. The Allegro in A Minor, D. 947, “Lebensstürme,” written in Schubert’s last year, is a cataclysm that owes much of its power to its form. Late in life, Schubert’s sonata movements expanded in a quite extraordinary way: Their willingness to wander—to make room for Schubert’s most sublime daydreams and his most upsetting nightmares—makes them unlike any music written before or since. This Allegro is no exception: Its tragic nature is laid bare with its opening gesture, a furious A-minor descent, and is somehow heightened rather than tempered by the unearthly beauty of its second theme. The arrival of this theme is a stunning event. In the work’s opening pages, Schubert uses the extra pair of hands at his disposal in a brilliant, unsettling way, creating constant rhythmic and motivic complication; unrest is everywhere. The second theme, by contrast, has the purity of a hymn, played pianississimo first in the distant key of A-flat major, then a celestial C major, exploring the uppermost reaches of the piano with unhurried wonderment. The central fact of this theme is not its beauty, nor even its profundity, but its faraway-ness. It is not another country, or another world, or even another solar system: It is unreality. It is as beautiful as music can be, but it does not bring comfort, because it exists only in Schubert’s imagination. The Allegro in A Minor is devastating on account of the disconnect between this utopia and a terrible fate that cannot be avoided and is growing closer with every iteration of the opening motif.

The Grande Marche No. 5 in E-flat Minor, D. 819, is something else: a funeral march as uncompromising as it is desolate, each bass note a step towards an inexorable fate. It is also a neglected masterwork—neglected even by the standards of Schubert’s four-hand music, a masterwork by any standard. In the most affecting way possible, it exemplifies one of Schubert’s signature qualities: the deep sorrow he finds in the major mode. If you know one thing, and one thing only, about diatonic music, it is this: Major keys convey happiness, minor keys sadness. It is an oversimplification, to be sure, but it is not wrong, per se.

Not wrong, that is, except with Schubert. For him, sorrow is a constant; it never goes away. It can be more or less deep, more or less mediated, more or less overt, but it is inescapable. And therefore, the appearance of the major mode—particularly in the context of minor-key music—often brings even greater pain. It is the pain that can be masked but never forgotten.

Following in the wake of this grim, remorseless, E-flat–minor march, the trio in the parallel major ought to offer relief; coming from Schubert’s pen and soul, it cannot. This music defies analysis: Its simplicity is extreme, and on the page, nothing in it looks likely to be moving. Yet it is moving beyond description: It cannot find its way to joy, but it offers such solace. It lays bare the harsh, heartbreaking realities of Schubert’s existence, and the role music played in making it bearable. Life is lonely, and death is inescapable. All there is, in the end, are E-flat minor and E-flat major—the former giving voice to the pain Schubert knew, the latter to the pain of what he longed to know.

By contrast, the Rondo in A Major, D. 951, contemporaneous to the Allegro and possibly intended as a companion to it, offers 15 minutes of nearly uninterrupted consolation. It spotlights that most elemental gift of Schubert: lyricism. Schubert’s character has a morbid streak running through it. It manifested from the time he was a teenager in works such as “Erlkönig,” and in the illness-plagued last years of his life, even in comparatively placid works, horrifying visions frequently intrude.

The A-Major Rondo is a work without horror; on this occasion, he manages to keep it at bay. The serenity and directness of this music are instantly disarming; it seems to have flowed from him utterly unimpeded. This being Schubert, that extraordinary serenity cannot quite mask the equally extraordinary fragility: Morbidity may have been an element of Schubert’s character, but Sehnsucht—longing—is its core. The generosity of this music is deeply moving of its own accord; when one considers the hurt and terror that defined his later life, making their way into so much of his later music and remaining just suppressed here, that generosity becomes overwhelming.

For all that, the Divertissement à la hongroise, D. 818, might be the most remarkable work on this program; certainly, it is the most unusual. Its power is, once again, inexplicable: There is no form of analysis that can help us understand why listening to it is such a peculiarly devastating experience. Its title is already deeply misleading. This massive work—its two outer movements, while formally ambiguous, are of symphonic proportions—is not a diversion or an entertainment. Rather, it is a portal into another world: a bleak, melancholy, terribly lonely world. This music is devastating but not depressing; it is too beautiful for that. Its point of origin is Hungarian folk music, which Schubert treats with the opposite of condescension: In his hands, even the most unassuming motif becomes unspeakably profound.

Whatever its ethnic roots, the Divertissement is unmistakably Schubertian. Schubert’s music contains the full spectrum of human emotion, to be sure. But loneliness was the central fact of his existence and, accordingly and heartbreakingly, it is at the center of his music as well. Schubert could convey what it is to be alone like no other artist, and he does so in nearly every one of his works, either fleetingly or unremittingly.

To experience this in the context of his four-hand music is to experience a deep irony, as there is no other musical genre in which two instrumentalists find themselves so close to one another, physically and otherwise. Often, the primo player’s left hand will cross below the secondo player’s right; nearly as often, Schubert will ask the two pianists to do the impossible and play the same note at the same time. (This might be an oversight; I am inclined to think that it is not, but rather a reflection of the human connection for which Schubert was so starved.)

But the closeness goes deeper than that. Many pianists have drawn an analogy between pedaling and breathing, in that both are done primarily instinctually, as needed. To have another pianist pedal for you, as you must in this music, is thus to allow another person to be your lungs. To trust that someone else will anticipate your choice to linger, or not, over a harmony that strikes you differently today from how it did yesterday is a profound sort of trust—the sort of trust that comes only with deep attunement to another human being.

That is to say: Playing Schubert’s four-hand music is an act of extreme intimacy in the service of music that conveys extreme loneliness. To feel Schubert’s loneliness so palpably is painful. But living with this music and being able to share it is a privilege and a gift.

—Jonathan Biss