This is a pair of WHITE Automatically Resetting Window Fall Prevention Devices. This pair will outfit 1 complete window. These Vision Industries Fall Prevention Devices also called Window Opening Control Devices (WOCD) work with Single and Double Hung Tilting & non-tilting Window Systems. These devices are required for use in all windows as specified in the 2012 International Building Code (IBC) & the 2012 International Residential Code (IRC). Vision Hardware is the only manufacturer in the United States making this product. This product has been extensively tested by an independent laboratory and exceeds the ASTM F2090 requirements. These devices are installed onto the window or window jambs using the hardware included. Once the devices are properly installed they will prevent the window sash from opening more than 4". In order to open the window farther, you must deactivate both devices using the single action mechanisms. The devices will automatically reset once the window is closed fully, once again limiting the sash from opening more than 4".