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 or pushing force, 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 and an unlocking position, 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 and said unlocking position 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 (21, 23, 24, 26, 27, 35, 61, 91) allowing a relative, such as a substantially linear, displacement 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 or pushing force, 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 and an unlocking position, 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 and said unlocking position 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 (21, 23, 24, 26, 27, 35, 61, 91) allowing a relative, such as a substantially linear, displacement 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.