Sketches are all too often used to make a simple description of the structural organization of Brian Ferneyhough's music. In order to go beyond this simplistic approach, the author has choosen to study and interpret a single page of sketches related to Kurse Schatten II for guitar solo. The title of the page, omitted in the score, first reveals a notable link between the work for guitar and the one for piano solo Lemma-Icon-Epigram. The same title also brings to light a distinctive feature of the "variations form" used in the second movement of Kurse Schatten II. The philological approach then allows the understanding and, above all, interpretation of how the rhythmic structures of that movement came into being. The page of sketches finally appears as the hidden place where inventiveness and arbitrariness clash with one another.