The stair tread mechanism is connected with a window lock and {HEAD ~ initial movement} of {TAIL ~ the stairs} unlocks the window 
The stair tread mechanism is connected with a window lock and { HEAD ~ initial movement } of { TAIL ~ the stairs } unlocks the window
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH