What Is Polyrhythm? Two Pulses at Once and the Ear Choosing

The vocabulary is used loosely and the distinctions are useful:

• •Polyrhythm proper means two or more conflicting pulse divisions sounding simultaneously, of which three against two is the simplest and four against three the next
• •Cross-rhythm is polyrhythm where the conflicting pulse persists against an established main beat rather than being a passing effect
• •Polymetre means parts in different metres with the same pulse, so that a five-beat pattern cycles against a four-beat one and they realign only after twenty beats, which is a different effect
• •Syncopation displaces accents within a single metre and is not polyrhythm at all, though it is frequently called that
• •An additive rhythm groups a long cycle unevenly, so that seven beats become three plus two plus two, which is standard in Balkan and Turkish music
• •The hemiola, a three against two relationship used in European music since the Renaissance, is the one form the classical tradition has always had, appearing throughout Baroque dance movements and in the well-known alternation in the song America from West Side Story

Each hand must maintain an independent regular pulse while the two coincide only at the start of the cycle, and the difficulty is that the brain strongly prefers to hear one pulse and fit everything to it. The standard teaching device is a mnemonic phrase whose syllables fall on the correct points, with the three-against-two pattern taught using a short phrase of the right rhythm, which works because language rhythm bypasses the counting. Musicians trained in traditions where this is standard learn it as children by playing in ensembles, where each player holds one part and the composite emerges from the group rather than from any individual, which is a considerably easier task than producing both parts alone. The bell pattern in West African ensembles functions as a timeline, a fixed asymmetric cycle that every other part orients against, and it does the job a conductor does in an orchestra without anyone being in charge.

In much of West and Central Africa, polyrhythm is the organising principle rather than an ornament. Ensembles of drums, bells and rattles each play a repeating pattern of different length and accent, and the music is the composite, which changes as parts shift relative to each other. The timeline patterns, including the widely distributed one usually written as a twelve-pulse bell figure, recur across an enormous geographical range. Those structures travelled with enslaved people to the Americas and underlie Cuban and Brazilian music, with the clave pattern functioning in salsa exactly as a timeline does, and every part in the ensemble either aligning with it or deliberately not. Indian classical music develops rhythmic complexity differently, through long cycles subdivided in mathematically elaborate ways and through the deliberate placement of a phrase so that it resolves on the first beat after crossing several cycles. Balkan traditions use additive metres. European classical music used the hemiola and otherwise kept a single pulse, which is the exception rather than the norm worldwide.

The absorption happened through several routes in the twentieth century. Jazz inherited African rhythmic organisation through the blues and through New Orleans, and developed it enormously, particularly from the 1950s in the work of drummers including Elvin Jones and Tony Williams, who played implied cross-rhythms against the pulse continuously. Classical composers took it up deliberately, with Stravinsky's shifting metres, Bartok's Bulgarian rhythms, Ligeti's piano studies built explicitly on African polyrhythm, and Conlon Nancarrow writing for player piano precisely because no human could execute the tempo relationships he wanted. Steve Reich studied drumming in Ghana in 1970 and built a body of work on phase relationships between identical patterns drifting apart. Progressive rock and metal adopted polymetre as a signature, and its use in contemporary popular music is now routine enough that listeners follow it without difficulty, which is the clearest evidence that the difficulty is a matter of training rather than of perception.

Polyrhythm means two conflicting pulse divisions sounding at once, of which three against two is the basic case, and it is distinct from syncopation, which displaces accents within one metre, and from polymetre, where parts of different lengths share a pulse. It is difficult to play alone because the brain insists on hearing a single pulse, and it is learned in ensembles where each player holds one part against a fixed timeline. It is the organising principle of West African and Afro-Caribbean music and reached Western art music through jazz and through composers who studied it directly.