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The cross-cap, or more precisely, the projective plane, can representthe subject of desire in relation to the lost object. A double loop drawn on its surface in effect divides this single-sided surface into two heterogeneous parts: a Möbius strip representing the subject and a disk representing object a, the cause of desire. The disk is centered on a point that is related to the irreducible singularity of this surface, which Lacanidentified with the phallus. Unlike the representation of the subject produced on the torus, here a single cut, which symbolizes castration, produces both the subject and the object in its divisions (figure 7).