This article aims at clarifying the nature of the “ideal elements” that Jean-Victor Poncelet introduced into geometry, notably in his Traité des Propriétés Projectives des Figures (1822). Moreover, it argues that the notebooks that Poncelet wrote in Saratov, as a war prisoner, between March 1813 and June 1814, shed light on the genesis of this concept and the part these elements played in Poncelet’s geometrical approach to geometry. In fact, the notebooks suggest that, as early as 1813-1814, Poncelet had shaped some of the key ideas that would turn the Traité into the foundation of projective geometry.