With the ID15 you get 2 door stopper alarms. Simply place one of the alarms behind any door. If an intruder attempts to open the door, the wedge-shaped design will prevent it from opening and activate a 100 dB alarm, scaring off the would-be intruder and alerting you to the attempted entry. These door stopper alarms come with a movement sensor. If anyone attempts to move the unit the alarm will immediately activate. (There is a 10 second delay before the movement sensor activates). When traveling, just toss these lightweight units in your luggage. The on / off switch will prevent them from accidentally sounding. This alarm requires two (2) AA batteries that are not included. They measure 6 inches x 1 and seven-eighth inches x 1 and five-eighth inches. Each box comes with two door stop alarms. For a single pack of this alarm please see item ID14 or for a door stop alarm with an adjustable movement sensor and streetwise security packaging please see item SDS85.