Dada, a cultural movement triggered by the anti-war sentiments of a group of European visual artists and writers during the First World War, promulgated provocative anti-art as a response to the prevailing complacent standards of art. By 1922 Dada had self-destructed, giving way to surrealism, a movement best known for the surprise, unexpected juxtapositions of its visual artworks and writings. Both movements have had a tremendous impact on all subsequent avant-garde painting, sculpture, theatre, film, poetry, and fiction. But such a claim of influence would seem senseless in contemporary music, where the presence of Dada and surrealism is generally unrecognized or forgotten.
However, these movements exerted a pervasive influence on 20th-century music, especially on mid-century avant-garde composers based in New York—among them Edgard Varèse, Stefan Wolpe, John Cage, and Morton Feldman. In addition, these composers paved the way for a Dada/surrealist aesthetic, a “normative Dada” whose radicalism was of an entirely different character than its European counterpart. The Dadaists themselves skirted madness and prison in the pursuit of an absolute aesthetic and a new social order whose raison d’être was the senseless carnage of World War I. The American scene was very different.
The aesthetic borders between Dada and surrealism are often difficult to distinguish and are additionally related to a third movement, Futurism, which began most notably in Italy, but also in England and Russia, which incorporated visual arts, literature, and music. Le bruit, noise with imitative effects, was introduced into art by the Italian Futurists Filippo Tommaso Emilio Marinetti (1876-1944) and Luigi Russolo (1885-1947), who used a chorus of typewriters, kettledrums, rattles, and pot covers to suggest the “awakening of the capital.” Richard Huelsenbeck, in his 1920 essay “En Avant Dada: A History of Dadaism” (Collected in Robert Motherwell’s anthology, The Dada Painters and Poets, Cambridge: Belknap, 1979), noted that Dadaists borrowed the term bruitism, or noise music, from the Italian Futurists.
There is a complex of interests, techniques, imagery, and intentions that characterizes surrealism in its search for epiphany. One of the most familiar and disturbing is a seamless conjunction of impossibilities in which objects that cannot exist in the same time and place are rendered with the perfection of reality, as in the paintings of René Magritte. Another is an interest in found objects in search of the revelatory image or moment that affords access to a “superior” reality. But these images also bring with them an antinomian charge; thus an interest in alchemy and magic. Dada, surrealism, and Futurist all applied an aesthetic of shock and a panoply of approaches in order to transcend the linearity of normal consciousness: free association, frottage, and automatic writing. Such was the path to the subconscious and the dream.
These movements paid close attention to advanced and developing technology, and the repetitive beauty of machines was a ubiquitous image. Contemporaneous technologies—film and photo montage, for example—provided a means of assaulting those laws of space and time that the subconscious impertinently refused to recognize. The machine was both a sign of modern urban power (the modern metropolis was another prominent obsession) and a locus of ambivalence about the Machine Age’s destructive and dehumanizing aspects, particularly after World War I. Machines also offered a new kind of sexual allure; that of the mechanical and repetitive. The products of machines—newspapers and other mass-produced items—took on similar associations, and this source material was put to work in the formation of a new poetics.
Dada had already attracted imaginatively anarchic musicians even before the end of the First World War. By far, the most inventive and radical musical constructions were those of Marcel Duchamp (1887-1968), who had taken up residence in the United States. Often working under the alias A. Klang (Klang: “sound” in German), his assemblages cunningly obscure the boundaries of text, music, representation, and notation a half-century before John Cage’s experiments in indeterminacy.
Duchamp’s Exercices de musique en creux pour sourds (“hollow [or vain] musical exercises for the deaf”) instruct the performer: “Given an agreed/conventional number of music notes, ‘hear’ only the group of those which are not played.” The first Erratum Musical (1913) (in the collection The Green Box) offers a randomly arranged sequence of 25 notes written on staff paper with treble and bass clefs and accidentals. There are no rhythmic indications to coordinate the parts. An arbitrary deployment of the dictionary definition of “to print” appears below the musical notations. The second Erratum Musical (also 1913), bearing the additional title “La mariée mise á nu par ses célibataires, même” (“The Bride Stripped Bare by her Bachelors, even”) uses a musical notation of only numbers and letters, without conventional music symbols, and with explanatory notes and diagrams. He writes, “the order of succession is interchangeable by whim,” and the piece is “unfinishable.” The second Erratum Musical is intended for “a specific musical instrument (player piano, mechanical organ, or other new instruments where the virtuoso intermediary is suppressed).”
These inventions are preludes to the provocations of Erik Satie (1866-1925), who was a crucial figure in the formation of surrealism. In a now-famous program note to Satie’s Parade (1917: produced by Diaghilev with Cocteau, Picasso, and Satie) Apollinaire coined the word, in referring to “…a sort of sur-realism [sur-realisme] which I see as the point of departure for a series of manifestations of that new spirit which promises to modify the arts and the conduct of life from top to bottom in a universal joyousness.” (Rollo Meyers, Modern French Music: Its Evolution and Cultural Background from 1900 to the Present Day. London: Blackwell, 1971, p. 20.)
But a surrealist “school” of composition never developed in France, undoubtedly because of the hostile indifference to music of the arch-surrealist André Breton, who believed that language had already subsumed and surpassed the possibilities of music.
However, as a result of the development of the tape recorder, a true surrealist music became possible in musique concrète—the art of “found” and manipulated sound. But tape recorders were not easily available until the late 1930s, coinciding with the decline of the surrealist movement. Still, surrealism served as a source of imagery for the aesthetic that grew up around this new technology. Symphonie pour un homme seul, a joint composition created by Pierre Schaeffer and Pierre Henry in 1950, was the culmination of a series of sonic experiments, including studies of the sounds of locomotives, pots, and turnstiles. Schaeffer himself remarked that his manipulations of recordings by looping (sillons firmés) were reminiscent of the first surrealist paintings. The aim was the creation of sound-objects, objets sonores, the result of a series of transformations of original material deformed beyond recognition, at a level where “the bell becomes a human voice, the voice a violin and the violin a sea bird.” (A la recherche d’une Musique Concrète, Paris: Éditions du seuil, 1952, p. 47.) However, musique concrète ultimately rejected the possibility of musical surrealism once the sonic results of the manipulations of found sounds became more important than the ability for listeners to discern these sounds’ original sources.
Edgard Varèse (1883-1965) arrived in the United States in 1915 and, although he maintained cultural connections to Europe, most of his works were composed in New York. The young Varèse had close ties with both Dadaists and Futurists, although he denied being of either camp. He hated being linked with art movements and was unusually secretive about influences on his work. Sometimes his caginess was excessive. Although his orchestral work Arcana bears a quotation from the alchemist Paracelsus, Varèse insisted that the passage did not inspire the work and that the work is in no way a commentary on it. However, surrealist imagery underlay what he claimed was an objective, science-based art.
Ultimately, Varèse was neither a Dadaist nor a surrealist but a kind of fellow traveler. In 1921 he signed a Dada manifesto opposing Futurism, “Dada soulève tout,” distributed in protest at one of Marinetti’s lectures. He also contributed poetry to the Dada review 391, and the poet Aragon described him as the “only composer of the Dada era.” Many of his instrumental works recall the machine language of the Dadaists. Elliott Carter describes this quality precisely:
Usually sharply defined, his rhythmic process recalls the clicking and rattling of rather complex machinery that seems to produce broken, out-of-phase cycles of sound. These rhythms shape the order of presentation of the notes of vertical harmonies that are frequently static and lead them to burst or explode in unexpected ways. (Elliott Carter, “On Edgard Varèse,” in The New Worlds of Edgard Varèse: A Symposium, ed. Sherman Van Solkema. ISAM Monographs #11: 1979, 2.)
Surviving material from Varèse’s unfinished opera, L’Astronome (1928), reveals the clearest evidence of a Dada aesthetic in his compositional output. But after an attempted collaboration with Robert Desnos and Alejo Carpentier, Varèse turned the project over to the playwright and theater visionary Antonin Artaud in 1933, who also failed to complete the libretto. (Eric Sellin, The Dramatic Concepts of Antonin Artaud. Chicago: U. of Chicago Press, 1968, pp. 71-75.)
Varèse’s treatment appears in Artaud’s complete works as Il n’y a plus de firmament. The action takes place in the year 2000. Dada and Futurist themes run riot; both Varèse and Artaud had gone beyond the provocations of Dada to envision sublime terrible worlds of their own. But they both spoke the Dada idiom: synesthesia, cataclysm, language beyond language, the world of machines, and the assault on all social and aesthetic boundaries. In L’Astronome, the audience was to be eradicated with a death ray (a rather unsatisfactory end for a bourgeois evening at the opera). Here are Varèse’s notes from 1928:
Discovery of instantaneous radiation – 30,000,000 times that of light. Rapid variation in the size of Sirius, which becomes a Nova. … Unexpected reception of signals – prime indivisible numbers 1,3,5,7. … During the catastrophes, it is [the scientists’] decision which turns the crowd’s rage against the astronomer …. Mysterious – in musical waves [ondes Martinot] (supple, fluctuating). Scientists study them. It is perhaps the acoustic language of Sirius. The brilliance of Sirius continues to augment, … precipitating catastrophes. Explosions, darkness, etc. … The crowds first attack the astronomer, later making a saint of him…. The crowd becomes rigid. The projector turns toward the room, blinding the audience. … A few wax mannequins look outwards, their eyes fixed and without expression.
Fernande Ouellette, Edgard Varèse (Paris: Editions, Paris, 1966, pp. 127-8.)
It is remarkable how closely Artaud’s apocalyptic visions, spatial imagination, and sense of the interrelated, constructive capacities of sound and light parallel so much of Varèse’s imagery. Artaud writes:
There is a concrete idea of music where sounds come into play like characters, where harmonies are cut in half and are mingled in with the precise interventions of words. Musical instruments are also to be objectified on the stage, and one must seek the means of producing new sounds: They will be used in the capacity of objects and as an integral part of the décor. Furthermore, the necessity of acting directly and profoundly on the sensitivity through the organs makes it advisable, from the viewpoint of sound, to seek out absolutely new qualities and vibrations of sound, qualities which contemporary musical instruments do not possess.
(Sellin, p. 84)
Compare this to Varèse:
[T]he new musical apparatus I envisage, able to emit sounds of any number of frequencies, will extend the limits of the lowest and highest registers, hence new organizations of the vertical resultants…. Not only the harmonic possibilities of the overtones will be revealed in all their splendor, but the use of certain interferences created by the partials will represent an appreciable contribution. … An entirely new magic of sound!
(Edgard Varèse, “The Liberation of Sound,” in Elliott Schwartz and Barney Childs, eds., Contemporary Composers on Contemporary Music
(NY: Holt, Reinhart & Winston, 1967, pp. 197-8.)
Where Varèse perceived sounds sweeping the audience like waves of light, Artaud desired a new illuminating apparatus capable of “projecting lighting in waves, or in sheets, or like a volley of flaming arrows.” And, where Varèse describes “sound projection” as a voyage into space,” Artaud claims that “[w]ords, too, have possibilities of sonorization, various ways of being projected in space.” (Sellin, pp. 86-7)
But it is in Varèse’s electronic music that all the changes of Dada and surrealism are rung, even though both of these movements were by now long on the wane. Poéme electronique, created for Le Corbusier’s Philips Pavilion at the 1956 Brussels World’s Fair, is a vehicle for the release of decades of shored up Dada/surrealist imagery. Recalling the distortion of scale that permeates surrealism, sound events in Poéme electronique emerge from silence, giving them the weight of icons. Pure electronic sounds as well as various “found” sound objects—bells, factory noises, distorted human voices, distorted conventional musical instruments, plus a quotation from Varése’s own Ionization—all are given the same experiential weight, surging from a void. The juxtaposition of these disparate materials achieve the violations of time and space that surrealism aimed at.
Stefan Wolpe (1902-1972) joined the Berlin Dadaists in 1920, just after poet and drummer Richard Huelsenbeck (1892-1974) had joined the group, having arrived from the Zurich Dadaists. Wolpe and his cohorts anticipated by more than thirty years works such as John Cage’s Imaginary Landscape IV for twelve radios, as well as the experiments of the concrètistes:
I had eight gramophones…at my disposal. And these were lovely record players, because one could regulate their speed.… [Y]ou could play a Beethoven Symphony very, very, slow, and very quick at the same time that you could mix it with a popular tune. You could have a waltz, then you could have a funeral march. So I put things together in what one would call today a multifocal way. …The concept of simultaneities is one of the most truly fascinating things.
Stefan Wolpe, “Lecture on Dada” (1962)
In 1929 Wolpe composed a setting of Kurt Schwitters’ Anna Blume for a tenor clown on a bicycle. After a period of intensive absorption in Schoenberg-influenced chromaticism, he dedicated himself to workers’ music, until he fled the Nazis and went to Vienna in 1933, where he briefly studied with Webern. But Dada stayed with him, as his lectures and theoretical articles attest. The concept of simultaneity was a crucial premise for Wolpe, particularly in his late-period works, and he is quite clear about the provenance of this concept. Describing his Enactments for three pianos (1950-53) he notes:
Many things are happening at the same time, curves hugely expanding, curves enormously contracting, a sound, a hit, a tone, a silence. These are not random situations, they’re highly calculated, but one experiences also the disparity of different qualities of events. … [Y]ou can have in this kind of music a kind which was also one of the early Dada obsessions, or interests, namely, the concept of unforeseeability, non-influence, non-directivity, you cannot explain. It means you cannot infer what is going to happen!” (“Lecture on Dada”)
In his 1960 lecture “On Proportions,” Wolpe proposed a mode of musical operations based on a vocabulary of symmetrical and asymmetrical pitch structures (i.e. proportions) grounded in the play of simultaneities in an abstract musical space. The second half of the lecture, however, conjures up a free-flowing, neo-Dada space in which musical proportional events could be assembled:
The proportions of the audible and the visible are twins of one and the same totality: the formal proof of unalterable nature, and of the nature of confused, inconsistent realities. The pathos of the unintentional arrangements of the wastepaper basket, the disorderly table—the general formal nature of the street scene, alienated synchronism, such pathos is the equivalent of a different, formally intended arrangement. Everything becomes the language of formal distinction, and the language is concrete, like the hieroglyphics of stones, or the distinct, engaged proportion of pitches.
Wolpe’s proportional system underlies the entirety of his late work (from about 1960 on). This considerable body of music is perhaps the most extensive to have been created on the basis of a Dada method. Wolpe’s lifelong commitment to Dada simultaneity predated by two decades John Cage’s pre-indeterminate percussion pieces of the early 1940s. The proportioning of musical works—”the proportioning of proportions” as Wolpe described it—was a common interest of Wolpe, Cage, and Morton Feldman.
THE SEEDS FOR A NORMATIVE DADA
The genealogy of an art movement follows approximately the same pattern as a political revolution. What was once a radical and even terrifying idea—universal suffrage, for example—eventually sheds its radical aura and assumes a middle class normality.
This was the fate of Dada/surrealism: its revolutionary impulse was domesticated even as its techniques stocked the instrumentarium of contemporary music. But many of its techniques fuelled the New York avant-garde: Bruitisme (noise music), machine/repetition esthetic, chance/stochastic selection, and disruption of scale (for example, Morton Feldman’s six-hour long string quartet). Also, its aura invested the New York scene with an anti-commercial radicalism hostile to traditional art and especially to commercialism.
There is a psychological chasm between the radical desperation of Tristan Tzara (1896-1963), for example, and the American optimism of John Cage (1912-1992), the classic American “tinkerer,” as he has so often been described. When Wolpe walked out of John Cage’s first “happening” at Black Mountain College, it signaled a schism between the European and American musical avant-gardes. Wolpe had strong objections to chance music; it offended his radical humanism, which relied precisely on the free and individual choice—the human fingerprint—that chance music strove to eliminate.
Dada’s mise en scène was the corpse-strewn landscape of World War I; Wolpe was an anti-Nazi agitator whose works were antifascist polemics. Witness his Battle Music, written at the height of World War II; which (according to Austin Clarkson) was a musical portrayal of Picasso’s Guernica. What could Wolpe have had in common with Cage’s Zen-flavored optimism? While the Dada-influenced works of post-war America were directed against a complacent commercialism, conformism, and cold-war ideological rigidity, looking through the writings and works of John Cage and the Fluxus group, one is struck by just how apolitical it all is.
As early as 1937, Cage was working with Merce Cunningham on dance pieces using noise elements, including found percussion instruments. In 1938 the first prepared piano piece appeared. The 1939, Imaginary Landscape No. 1 featured piano, cymbals, and turntables playing test tone recordings at different and changing speeds. Cage’s work at this point could be assimilated to both Dada and Futurist models (for example, the intonarumore of Russolo). But from the mid ’40s on, Cage fell under the influence of non-Western and “alternative” philosophies—first Indian philosophy, then Meister Eckhart, and finally the Zen aesthetic of silence. In Cage’s 1950 Lecture on Nothing, he stated, “I have nothing to say and I am saying it and that is poetry as I need it.”
Buddhism gave Cage a philosophical framework in which to rework Duchamp’s chance techniques into an aesthetic of pure sound. His credo—”embrace whatever comes along”—was a product of Zen studies, but the idea owes as much to the simultaneity of Dada and related practices as it does to Buddhism. This was the period of his study of the I Ching, which he used as a generator for chance operations aiming at a complete negation of the composer’s will, “free of individual taste and memory.” The 1951 Imaginary Landscape No. 4 for 12 radios, although radical in its time and place, was, as we have seen, preceded by the Dada assaults on Beethoven that Wolpe describes above. In his 84 Music for Piano compositions (1952-56), imperfections on ordinary paper were transformed into pitches by adding staff lines and clefs, recalling Duchamp’s assemblages of 1913.
If Cage could be construed as a latter-day musical Dadaist, Morton Feldman (1926-1987)—who had studied composition with Wolpe before aesthetically aligning himself with Cage, could be considered his surrealist counterpart. Throughout his life Feldman maintained close personal ties to visual artists, and even some of his titles suggest a surrealist aesthetic: Vertical Thoughts III (1963), Madam Press Died Last Week at 90 (1970), I Met Heine on the Rue Fürstenberg (1971), Crippled Symmetry (1983). His public persona—that of a bluff Jewish Walt Whitman—belies the fragility and nostalgia so characteristic of his music; just as one would not expect that the composer of a six-hour long string quartet could compose miniatures that bring to mind Joseph Cornell’s intimate boxes of found objects. In a certain sense Feldman, like Cornell, was a miniaturist. His characteristic manner is to reiterate a particular gesture in a series of variants whose voicings, pitch content, and shape gradually change over the course of time. The silences separating these “views” allow the listener to experience each of them as a singular moment in the manner of a late Wolpe work; although at a radically slower tempo.
Much of Feldman’s music was composed at a time when music theory extolled transparent and totalizing structure; serial composition fulfilled these expectations. From such a viewpoint, Feldman’s music was “un-analyzable.” Such an approach could never penetrate the surface of his music, whose essence lies in the specific character of sounding events, and not in their abstract organization. But there is still much to be said about the music from an analytic viewpoint. Steven Johnson’s excellent article on Rothko Chapel (1972) carefully describes the relationship between the Mark Rothko paintings, the architecture of the chapel housing them, and Feldman’s composition, all of which were commissioned by John and Dominique de Menil. Johnson applies set-class analysis to the pitch structure of the work, which he understands as a “network of pitch-class adjacent dyads or trichords, spaced so that they interlock or nest within each other. (Steven Johnson, “Rothko Chapel and Rothko’s Chapel,” Perspectives of New Music 32/2, 1994, pp. 18-20.)
But this misses an important aspect of Feldman’s technique: its indebtedness to Wolpe’s principles of symmetry and asymmetry, his careful control over the rate of chromatic circulation (noted by Johnson) and equally careful placement of sounding events in musical space. Feldman was perhaps even more sensitive than Wolpe to the subtle spatial motions that can be achieved through re-voicings of events.
NORMATIVE DADA’S LEGACY: FLUXUS & EARLY MINIMALISM
There is an element of historical re-enactment in the Cage-influenced Fluxus movement, which began in the early ’60s and continued through the ’90s. The Lithuanian-born artist George Maciunas (1931–1978) organized the first Fluxus event in 1961 at the AG Gallery in New York and the first Fluxus festivals in Europe in 1962. The idea of the “event” originated in John Cage’s class at the New School, which led to the performance scores for “Fluxconcerts.” Participants included artists George Brecht, Al Hansen, Allan Kaprow, and Allison Knowles. Other Fluxus artists included Joseph Beuys, Nam June Paik, and La Monte Young.
But Fluxus distinguished itself from Dada in its preference for Zen-influenced simplicity, for example in much of the work of Yoko Ono. Ultimately, there were a number of strategies at work in the Fluxus movement: Dada provocations, as in George Maciunas’s Solo For Violin or Larry Miller’s Patina; Koan-like works such as Yoko Ono’s Tape Piece I and Fly Piece; Theater of the Absurd events like Ono’s Wall Piece for Orchestra to Yoko Ono and Ken Friedman’s Family Planning Event. (Note the haiku-like typography.)
George Maciunas: Solo for Violin
Old Classic is performed on a violin.
where pauses are called, violin is
mistreated by scratching the floor with it,
dropping pebbles through f hole, pulling
out pegs, etc.
Larry Miller: Patina
urinate on an egg until it has a nice
patina or until it explodes.
Yoko Ono: Tape Piece I
Take the sound of the stone aging.
Yoko Ono: Fly Piece
Yoko Ono: Wall Piece for Orchestra to Yoko Ono
Hit a wall with your head.
Ken Friedman: Family Planning Event
Get pregnant for 18 months and have
(For more details on Fluxus performance works, see the Fluxus performance research e-publication 2002 edited by Ken Friedman, Owen Smith, and Lauren Sawchyn.)
Another manifestation of late-20th-century cultural re-enactment can be found in both American and European minimalism. Minimalist music was foreshadowed by the asymmetrical reiterative gestures in Stravinsky’s neoclassical appropriations from the 1920s, which also foreshadowed the normalization of Dada/surrealism. But there are also other antecedents of this style which point even more directly to aesthetic tendencies that are clearly aligned to Dada and surrealism.
For example, Steve Reich’s early music recalls “machine” Dada (Pendulum Music 1968), appropriation—both of African-American speech (It’s Gonna Rain, 1965 and Come Out, 1966) and of third-world musical culture (Drumming 1970-71). While Reich has gone on to create an expressive, even a decidedly personal music—with at least one work (the gorgeous 1981 Tehillim) even hinting at that bête noir of postmodernism, aesthetic transcendence, the technical apparatus of a typical Reich piece often proceeds by attrition, a process of dialectical transformation, which also recalls Dada/surrealist disruptions of scale and time. In Piano Phase in particular, the listener’s time-sense is effectively blurred by its gradual transformations.
A common first reaction to early minimalist works was to condemn them as machine-like and lacking soul; the digital age seems to accept them as machines with soul. This is another case of normative Dada. After all, original Dada’s sadomasochistic fascination with the robotic, machine-like, and repetitive was an expression of horror; but far from manifesting repulsion, minimalism focuses on the hypnotic and lyrical potential of reiteration.
Normative Dada is a common dialect in our current musical language, tamed by the blandishments of mass culture. It offers a whiff of danger, bohemianism, and a radical genealogy. It is highly compatible with progressive politics and the postmodernist blurring of high and low culture. But it is not a revolutionary movement; it is the result of a normalization process. Of course, many individuals in the United States have taken great aesthetic and political risks, hitting rock-bottom in mental institutions or getting jailed for their troubles (the Beats offer a number of examples). But music is weighted differently than other arts. The expense of musical production, a resolutely middle-class training process for musicians, and a two-thousand-year-old heritage are hardly conducive to revolution; nor should they be.
America is a great sponge for radicals; it attracts them, then absorbs them, and even the radical art communities of the ’50s and ’60s have dissolved. This is not to claim that either politicization or institutionalization are desiderata of artistic expression. But it is worth remembering that Dada concealed a significant dose of idealism. Given our current political and cultural situation, this bit of provocation is very much in order.
Matthew Greenbaum‘s compositions have been presented/performed/commissioned by the Darmstadt Summer Festival, the Leningrad Spring Festival, the JakArt Festival (Indonesia), Hallische Musiktage, the Fromm Foundation, Da Capo Chamber Players, Cygnus, Parnassus, Fred Sherry, Marc-André Hamelin, David Holzman, Stephanie Griffin, the Talea String Quartet, Network for New Music/Penn Council on the Arts, the Group for Contemporary Music, Orchestra 2001, and the Houston Symphony. Recordings appear on CRI, Centaur and New World and a new all-Greenbaum CD is scheduled for release on the Furious Artisans label next year. Greenbaum studied composition with Stefan Wolpe and Mario Davidovsky and was artistic director of the Stefan Wolpe Centennial Festival NY (2002-3). He is currently a professor of composition at Temple University.Sign up for our monthly NewMusicBox newsletter