"If the rhythms or combinations of African rhythms have appeared to some musicologists so difficult that they seemed to defy any analysis, it is by ignorance of the processes of which they are made. It is indeed impossible to understand if we only hear it through a recording, a polyrhythm whose elements can not be isolated by attributing them to the different instruments played together ". "Reduced to an isolated instrument, the rhythmic formula is always simple. It is from the combination of these formulas that results, as a result of an interweaving whose principle or principles remain to be clarified, the rhythmic complication seemingly incomprehensible "wrote Gilbert Rouget, eminent specialist of African music, in 1961. Simha Arom has designed and developed a decryption method that makes it possible precisely to isolate each constituent part of a polyrhythmic assembly without necessarily desynchronizing it from all the others. This method allows the transcription and analysis of polyrhythms, but also polyphonies, whether vocal or instrumental. The book includes, in addition to the presentation of methodological and conceptual tools, a foray into time using written testimonies (explorers, travelers, missionaries) dating back to the fifteenth century. An important section is devoted to the structuring of time in African music.