Window design and construction must be carefully considered wherever there is a risk that windows more than two metres above ground level may be opened by unauthorised people and there is a risk of them climbing out of or falling from them. This is particularly the case in schools, nurseries, residential homes, care homes, hotels, residential accommodation and similar places. Where these risks exist British Standard 8213 requires the fitting of a safety restrictor, a mechanical device that limits the initial opening of a window to no more than 100mm (four inches). Once provided the integrity of each device must be maintained. Restrictors can usually be overridden by a key or other “child proof” mechanism for cleaning; they must be reinstated once the work has been completed. Sometimes a second device will be required. This will be for example, when people have been seen interfering or are known to have interfered with window restrictors. See also; Glass; Glazing.