Windows are left unlocked at a much higher rate than doors.
An open window that is visible from the street may be the sole reason that a house is targeted. An open window with only a locked window screen is particularly inviting to thieves. Access is quick, easy and silent.
Windows have latches, not locks; they should have secondary locking devices.
Wooden sticks/dowels work well with horizontal sliding windows.
For vertical windows, through the frame pins work well.
For ventilation, leave no more than a 4 – 6 inch window opening.
Make sure that opening is not large enough to allow someone to reach through to unlock the door or remove window lock.