An apron slat of a system for closing an opening of a building, said slat: - includes, on the one hand, a slat stationary part, on the other hand, a slat movable part mounted pivoting relative to the slat stationary part, between a retracted position and a deployed position and, furthermore, control means to control the pivoting of the slat movable part relative to the slat stationary part, under the impulse of another following slat or another preceding slat, these control means including a knuckle including, on the one hand, an upper portion that is secured in motion with the slat movable part and that is mounted pivoting relative to the slat stationary part, on the other hand, a lower portion, that is designed to be mounted pivoting relative to the other following slat, and that is mounted articulated relative to the upper portion of the knuckle; - is characterized in that: - the upper portion of the knuckle is mounted relative to the slat stationary part, pivoting around an upper pivot axis; - the lower portion of the knuckle is designed to be mounted relative to the other following slat, pivoting around a lower pivot axis parallel to the upper pivot axis; - the lower portion of the knuckle is mounted relative to the upper portion of this knuckle, articulated around an articulation axis extending, on the one hand, parallel to the upper pivot axis and the lower pivot axis and, on the other hand, offset laterally relative to a plane in which this upper pivot axis and this lower pivot axis extend; - the slat includes securing means for securing the movement of the slat movable part and the upper portion of the knuckle, these securing means including, on the one hand, nesting means included by the upper portion of this knuckle and, on the other hand, complementary nesting means, included by the slat movable part, and which cooperate by nesting with the nesting means. - includes, on the one hand, a slat stationary part, on the other hand, a slat movable part mounted pivoting relative to the slat stationary part, between a retracted position and a deployed position and, furthermore, control means to control the pivoting of the slat movable part relative to the slat stationary part, under the impulse of another following slat or another preceding slat, these control means including a knuckle including, on the one hand, an upper portion that is secured in motion with the slat movable part and that is mounted pivoting relative to the slat stationary part, on the other hand, a lower portion, that is designed to be mounted pivoting relative to the other following slat, and that is mounted articulated relative to the upper portion of the knuckle; - is characterized in that: - the upper portion of the knuckle is mounted relative to the slat stationary part, pivoting around an upper pivot axis; - the lower portion of the knuckle is designed to be mounted relative to the other following slat, pivoting around a lower pivot axis parallel to the upper pivot axis; - the lower portion of the knuckle is mounted relative to the upper portion of this knuckle, articulated around an articulation axis extending, on the one hand, parallel to the upper pivot axis and the lower pivot axis and, on the other hand, offset laterally relative to a plane in which this upper pivot axis and this lower pivot axis extend; - the slat includes securing means for securing the movement of the slat movable part and the upper portion of the knuckle, these securing means including, on the one hand, nesting means included by the upper portion of this knuckle and, on the other hand, complementary nesting means, included by the slat movable part, and which cooperate by nesting with the nesting means. - the upper portion of the knuckle is mounted relative to the slat stationary part, pivoting around an upper pivot axis; - the lower portion of the knuckle is designed to be mounted relative to the other following slat, pivoting around a lower pivot axis parallel to the upper pivot axis; - the lower portion of the knuckle is mounted relative to the upper portion of this knuckle, articulated around an articulation axis extending, on the one hand, parallel to the upper pivot axis and the lower pivot axis and, on the other hand, offset laterally relative to a plane in which this upper pivot axis and this lower pivot axis extend; - the slat includes securing means for securing the movement of the slat movable part and the upper portion of the knuckle, these securing means including, on the one hand, nesting means included by the upper portion of this knuckle and, on the other hand, complementary nesting means, included by the slat movable part, and which cooperate by nesting with the nesting means.