The First Alert Alarm is great for every door and window in the house. A study found that the average home has at least 16 doors, windows, and entry points and some larger homes have many more. The First Alert® Alarm adds a level of security to virtually any door or window. When the alarm is triggered, a piercing ultra decibel sound is emitted indicating that your security may have been breached. It installs easily. There's no tools or measuring required. Simply peel off the adhesive backing on the alarm unit and stick it in place. By placing an order you will be upgraded to the 3 way unit. The switch of the 3 way unit has an alarm setting, an off setting and a chime setting. When set, the chime setting allows you to hear a single pleasant tone when a door or window is opened. Product includes: First Alert Alarm (Set of 4 Alarms).