Abstract: An apparatus for preventing unwanted entry through the windows of a structure includes an outer frame which is fastened to the structure and an inner frame which is connected to the outer frame by a pair of hinges and by a lock assembly. The lock assembly includes a locking rail which is slideably mounted in a vertical member of the inner frame and which slides downward under the influence of gravity to a normally locked position in which portions of the locking rail engage a pair of T bolts which are mounted on the outer frame and which project into the vertical member of the inner frame. The locking rail includes a pivotally mounted pin which must be moved to a vertical position to unlock the inner frame.