Ideally, they get placed on every door and window in the house, and they help save money as well. As they work usually – with proper maintenance – when a window or door opens or is breached in the time of the alarm it will send a signal to the control panel to trigger an alarm. That notifies us of the action, also in which room it opened. In that way, for example we don't need to check the whole house around what was left open or what needs to be closed, just a quick check on a panel and a single contact to do what is desired, without even going there.