at spaced intervals along the respective scissor linkages 40 41 each scissor linkage 40 and 41 includes a plurality of pivotally interconnected scissor arms 42 each having the aperture 45 centrally located and sized to prevent reverse passage removal of 39 and 39 39 therethrough once attached u00b0 extension and retraction of the first scissor linkage 40 of the first traveler assembly 30 serves to move the plurality of louver carriers 32 and attached first set of individual louvers 50 along the guide rails 27 and 27 39 of the first channel 25 extension and retraction of the scissor linkage 41 of the second traveler assembly 31 serves to move the plurality of louver carriers 32 and attached second set of individual louvers 60 along the guide rails 28 and 28 39 of the second channel 26 control means is provided for selectively and individually extending and retracting the scissor linkages 40 and 41 of the first and second traveler assemblies 30 and 31 along the length of the header casing 20 so that either the first set of individual louvers 50 or second set of individual louvers 60 may be pulled open or closed while the other remains stationary the control means includes a cord 70 for controlling the first set of louvers 50 extending through the first apertures 43 of all the stems 33 from one end of the header casing to the other and back through the second aperture 44.