In the year 1536, the world of music theory underwent a quiet revolution that would redefine how humanity hears the very fabric of sound. Before this date, the distinction between consonance and dissonance was a rigid binary, a cosmic law where certain intervals were inherently sweet and others inherently harsh. However, the German composer and music theorist Paul Hindemith would later observe that for a thousand years, the definitions of these terms had varied so wildly that they had never been completely explained. This uncertainty is not a modern confusion but a historical constant, rooted in the physical reality of how sound waves interact within the human ear. When two notes are played simultaneously, they do not merely exist side by side; they collide. If their frequencies are close enough to create a rapid fluctuation in amplitude, the ear perceives a beating or roughness that the brain interprets as tension. This physical phenomenon, known as sensory dissonance, is the biological engine behind the musical concept of dissonance. It is not a cultural construct alone, but a physiological response to the interference of sound waves within the critical band of human hearing. The brain, wired to recognize patterns, flags these irregularities as stress signals, triggering an EEG pattern known as the P300 response. This biological alarm system is the reason why a tritone can feel like a scream and a perfect fifth feels like a sigh, establishing a biological foundation for what would become the most debated concept in Western music history.
Ancient Ratios and Medieval Blends
The story of consonance begins not with music, but with mathematics in Ancient Greece, where the philosopher Pythagoras discovered that the intervals of the fourth, fifth, and octave corresponded to simple numerical ratios of string lengths. These ratios, such as 2:1 for the octave and 3:2 for the fifth, were considered the only true consonances, while all other intervals were labeled diaphonos, or discordant. This mathematical purity held sway for centuries until the early Middle Ages, when the Latin scholar Boethius described consonance as the blending of a high sound with a low one, arriving sweetly and uniformly at the ears. By the 13th century, the theorist Johannes de Garlandia had expanded this system to include thirds and sixths, though he still classified them as imperfect consonances that required resolution. In the music of Guillaume de Machaut, the great composer of the 14th century, the minor seventh and major ninth were treated as harmonic consonances because they correctly reproduced the interval ratios of the harmonic series, softening their effect. This was a stark contrast to the modern ear, which hears these intervals as tense and unstable. The medieval mind did not hear the tritone as a devil's interval, but rather as a deponent sort of fourth or fifth, sometimes stacked with perfect fourths and fifths to create resonant units. The concept of resolution was not a beat-to-beat necessity as it would become later; instead, the music flowed through a succession of non-consonant sonorities, limited only by the next cadence. This era established a fluidity where the minor seventh and major ninth were fully structural, and the thirds and sixths were not the intervals upon which stable harmonies were based, creating a soundscape that feels alien to the modern listener accustomed to the dominance of the major and minor triad.
The transition from the medieval to the Renaissance period brought a radical shift in the treatment of the perfect fourth, which was suddenly elevated to the status of a dissonance when placed above the bass. This change forced composers to adhere to the regola delle terze e seste, or the rule of thirds and sixths, which dictated that imperfect consonances must resolve to a perfect one through specific voice-leading progressions. The anonymous theorist of the 13th century had allowed sequences of three or four imperfect consonances, but by the 15th century, the theorist Anonymous XI permitted four or five successive imperfect consonances, signaling a growing tolerance for complexity. Adam von Fulda, writing in the 16th century, noted that while the ancients had forbidden sequences of more than three or four imperfect consonances, modern composers did not prohibit them. This period marked the beginning of a tension between the desire for variety and the need for stability, as composers began to explore the expressive potential of intervals that were previously considered too harsh. The minor seventh and major ninth remained structural, but the thirds and sixths began to take on a new role, no longer merely passing tones but essential components of the harmonic fabric. The concept of resolution became more immediate, with the fourth and fifth cadences needing to be the target of resolution on a beat-to-beat basis. This shift laid the groundwork for the Common Practice Period, where the preparation and resolution of dissonance became a fundamental rule of composition. The music of this era was a delicate balance between the ancient mathematical ratios and the emerging desire for emotional expression, creating a soundscape that was both intellectually rigorous and emotionally resonant.
Bach's Agony and Mozart's Daring
In the Baroque era, composers like Johann Sebastian Bach began to use dissonance not merely as a structural necessity but as a powerful tool for emotional and religious expression. At the end of the St Matthew Passion, Bach inserted an unexpected and almost excruciating dissonance over the very last chord, where the melody instruments insisted on B natural, the jarring leading tone, before eventually melting into a C minor cadence. This moment, heard by conductor John Eliot Gardiner as a final reminder of the agony of Christ's betrayal, demonstrates how dissonance could be used to convey the deepest human suffering. In the opening aria of Cantata BWV 54, Widerstehe doch der Sünde, nearly every strong beat carries a dissonance, creating a picture of desperate and unflinching resistance to the Christian to the fell powers of evil. Albert Schweitzer described this aria as beginning with an alarming chord of the seventh, meant to depict the horror of the curse upon sin. The Common Practice Period saw the emancipation of the dissonance, where composers like Wolfgang Amadeus Mozart began to experiment with radical dissonances that challenged the listener's expectations. Mozart's Quartet in C major, K465, known as the Dissonance Quartet, opens with an adagio introduction that gives the work its nickname, featuring passing dissonances that are implied rather than sounded explicitly. The A flat in the first bar is contradicted by the high A natural in the second bar, creating a false relation that does not sound together as a discord but creates a sense of tension. Mozart's Piano Concerto No. 21, the Elvira Madigan, contains a magical passage in the slow movement where subtle but explicit dissonances on the first beats of each bar are enhanced by exquisite orchestration, creating a feeling of discomfort that makes the hearers sit up. These moments of dissonance were not errors but deliberate choices, designed to evoke a specific emotional response and to push the boundaries of what was considered acceptable in music.
The Horror Fanfare and the Modern Break
The 19th and 20th centuries witnessed a dramatic escalation in the use of dissonance, as composers like Ludwig van Beethoven, Richard Wagner, and Igor Stravinsky began to treat dissonance as a primary vehicle for dramatic expression. The finale of Beethoven's Symphony No. 9 opens with a startling discord, consisting of a B flat inserted into a D minor chord, which Roger Scruton alludes to as a huge Schreckensfanfare, or horror fanfare. When this passage returns later in the same movement, the sound is further complicated with the addition of a diminished seventh chord, creating what Scruton calls the most atrocious dissonance that Beethoven ever wrote. Wagner took this further in his opera Götterdämmerung, where the scene known as Hagen's Watch conveys a sense of matchless brooding evil through excruciating dissonance in bars 9, 10, described as a semitonal wail of desolation. The early 20th century saw the culmination of this trend in Gustav Mahler's unfinished 10th Symphony, where the Adagio opens with a diminished nineteenth, a searingly dissonant dominant harmony containing nine different pitches. Richard Taruskin noted that this chord, which contained nine different pitches, would have been called unprecedented cacophony by the music theorist Guido Adler. The year 1913 marked a turning point with the premiere of Igor Stravinsky's The Rite of Spring, where the Sacrificial Dance featured a rhythmic and harmonic language that shattered the conventions of the past. This work, along with the experiments of Henry Cowell, who viewed tone clusters as the use of higher and higher overtones, signaled the emancipation of the dissonance. The tritone, once considered the most dissonant interval, was re-evaluated, and the minor ninth was placed as the most dissonant interval of all by theorists like Dan Haerle. The modernist movement had effectively promoted the major ninth and minor seventh to a legitimacy of harmonic consonance, creating a new soundscape where the boundaries between consonance and dissonance were blurred and redefined.
The Jazz Concept and the Critical Band
In the mid-20th century, the theoretical understanding of consonance and dissonance underwent a profound transformation with the work of George Russell and his Lydian Chromatic Concept of Tonal Organization. Russell presented a view that returned to a Medieval consideration of harmonic consonance, arguing that intervals when not subject to octave equivalence and correctly reproducing the mathematical ratios of the harmonic series are truly non-dissonant. This theory granted consonance to the septimal minor seventh, major ninth, neutral eleventh, neutral thirteenth, and diminished fifteenth, pitches that exist only in a universe of microtones smaller than a halfstep. Russell extended the virtual merits of harmonic consonance to the 12TET tuning system of Jazz and the 12-note octave of the piano, granting consonance to the sharp eleventh note, the sole pitch difference between the major scale and the Lydian mode. Dan Haerle, in his 1980 The Jazz Language, extended this idea further, placing the minor ninth as the most dissonant interval of all, more dissonant than the minor second to which it was once considered by all as octave-equivalent. Haerle also promoted the tritone from the most-dissonant position to one just a little less consonant than the perfect fourth and perfect fifth. This theoretical shift reflected a broader cultural change, where the distinction between consonance and dissonance was no longer a fixed binary but a fluid spectrum. The concept of the critical band, which describes the frequency range within which the ear processes sound, became a key factor in understanding how dissonance is perceived. The interaction of partials within the critical band determines the degree of beating and roughness, which in turn influences the listener's perception of consonance or dissonance. This scientific understanding of the ear's response to sound provided a new framework for composers and theorists to explore the expressive potential of dissonance, leading to the development of new musical styles and techniques that challenged the traditional boundaries of harmony.