Making a window more secure
A thin blade slipped between the sashes will open a thumb latch. Even a keyed latch pries apart easily.
You can secure a window better with a simple metal pin, sold by locksmiths. that slips into a hole in the sashes’ meeting rails. You recess the pin 1/8 inch and retract it with a wire clip that comes with the pin. You can use a clipped-off steel nail in similar fashion and draw it out with a magnet. Another device is a bolt that you insert and remove with a winged key.
Install these devices in the meeting rails’ corners. Then open the window 31/2 inches and put in a second pair so that you can lock it open. Stay 1/2 inch away from the glass and edges.
You can get a keyed lock that projects a rod through the sashes or one that mounts on top of the lower sash and projects a rod into holes in the top sash. Another locks to a plate on the top sash and presses against the lower sash. Similar units are available for metal-framed and sliding windows.
On a casement window, replace the latch with a locking latch-either a right- or a left-hand model.
Install locks with nonretractable, one-way screws. Keep a key out of sight and far enough away so that an intruder can’t fish it out. Never put a keyed lock on a fire-exit window.