A displacement system for opening and closing a building aperture cover such as a window or a door, wherein said displacement system comprises: - a drive system comprising a drive unit , - an actuator system comprising a push-pull chain , wherein the push-pull chain is configured to be driven by the drive system to be retracted or extended, and a locking part displacer connected to said push-pull chain and arranged to be displaced relative to said push-pull chain when subjected to a pulling force (F1) or pushing force (F2), wherein said displacement is in the direction of said pulling force or pushing force, - one or more locking parts movably connected to said actuator system, wherein said one or more locking parts is/are configured to move between a locking position (POS1) and an unlocking position (POS2), wherein said one or more locking parts are connected to said locking part displacer so that the one or more locking parts is/are displaced between said locking position (POS1) and said unlocking position (POS2) when the locking part displacer is displaced relatively to said push-pull chain , whereby an engagement part of the locking part is displaced in a direction transverse to said subjected force (F1, F2), wherein one or more chain linking members connects said locking part displacer and said push-pull chain by one or more displaceable connections allowing a relative, such as a substantially linear, displacement (M1) between the one or more chain linking members and said locking part displacer , wherein a housing arrangement is configured to house and cover said drive system and a folded part of the push-pull chain when the push-pull chain is retracted, and wherein at least a part of said one or more locking parts is/are configured to move out of a housing opening of the housing and into the housing opening when said drive system operates said push-pull chain . - a drive system comprising a drive unit , - an actuator system comprising a push-pull chain , wherein the push-pull chain is configured to be driven by the drive system to be retracted or extended, and a locking part displacer connected to said push-pull chain and arranged to be displaced relative to said push-pull chain when subjected to a pulling force (F1) or pushing force (F2), wherein said displacement is in the direction of said pulling force or pushing force, - one or more locking parts movably connected to said actuator system, wherein said one or more locking parts is/are configured to move between a locking position (POS1) and an unlocking position (POS2), wherein said one or more locking parts are connected to said locking part displacer so that the one or more locking parts is/are displaced between said locking position (POS1) and said unlocking position (POS2) when the locking part displacer is displaced relatively to said push-pull chain , whereby an engagement part of the locking part is displaced in a direction transverse to said subjected force (F1, F2), wherein one or more chain linking members connects said locking part displacer and said push-pull chain by one or more displaceable connections allowing a relative, such as a substantially linear, displacement (M1) between the one or more chain linking members and said locking part displacer , wherein a housing arrangement is configured to house and cover said drive system and a folded part of the push-pull chain when the push-pull chain is retracted, and wherein at least a part of said one or more locking parts is/are configured to move out of a housing opening of the housing and into the housing opening when said drive system operates said push-pull chain . <EOS>