which respective window panels 18 20 are mounted each window panel being a pane of glass or transparent plastic longitudinally extending segments of the window frame 12 include a guide rail 13 on which at least the first window unit 14 is mounted on the guide rail 13 for horizontal slidable movement therealong although both the first 14 and second 16 window units are preferably slidably mounted for movement therealong fig2 a of course the window frame 12 may include other stationary window panes 22 respective secondary window frames include first 24 and second 26 vertical frame members the vertical frame members are configured to be superimposed over one another when the window units are moved to a closed configuration more particularly the second vertical frame member 26 includes an outer segment 28 and an inner segment 32 joined together to form a channel having a female configuration capable of receiving the first vertical frame member 24 therein fig3 b therefore the first vertical frame member 24 is essentially captured by the second vertical frame member 26 at the closed configuration it should be observed that the inner segment 32 is only necessary relative to the locking assembly as to be described more fully below inner 32 and outer 28 segments of the second vertical frame member 26 define through holes 34 30 respectively fig4 a in addition the first vertical frame member 24 defines a through hole 25 fig2 b when the first 14 and second 16 window units are at the closed mated configuration all of the through.