The style of the underdrawing in some triptychs by Hieronymus Bosch was studied. <i>(Garden of Earthly Delights, The Prado's Haycart, the Bruges Last Judgment)</i>. The similarities in draftmanship and painting were assessed and several important <i>pentimenti</i> were observed. The question of the homogeneity of Bosch's style was raised. We could approach the chronology of Bosch's work as follows: several types of underdrawing can be observed in a single painting; consequently, the various types do not correspond to a certain chronological order; but there seems to be a relationship between the type of underdrawing on the one hand, and the type of pictorial execution and style on the other hand. Therefore it is impossible to establish a chronology for Bosch's work by taking style (pictorial execution, etc.) as a basis. In our opinion, it is impossible to reasonably state the question of the evolution of Bosch's work since we have no historical document relative to a still existing or an hypothetically identifiable painting. -- AATA