A corner accessory for placing at the junction between at least two ducts that extend in different directions, includes two distinct flaps, one of which has a projecting element and the other of which includes an elongate opening, the projecting element being engaged in the elongate opening so as to move along a longitudinal direction of the opening to allow the flaps to move relative to each other. According to the invention, the elongate opening has a longitudinal edge parallel to the longitudinal direction and opening out locally to a slot that opens out into a free edge of the flap.