This book attempts to solve the yet unsolved mystery: what are the intuitive rules of versification followed by poets and readers in writing and reading poetry? It performs a small Copernican Revolution, shifting the constraints from verse structure to the reader's "Rhythmic Competence". It confers a psychological meaning upon the phrase "rhythmical performance", that allows us to make predictions concerning the rhythmical performance of a metrically complex line. The theoretical discussions are supported by empirical research, in which readings by leading British actors and colleagues from the academy are submitted to an instrumental investigation.