This compact, attractive MAGNETIC DOOR ALARM uses magnetic affinity to initiate its 110 decibel siren. The base of the alarm is attached to door or window frame, the actuator is connected to the window or door. The alarm is activated by opening the door or window. Great for any door or window, particularly those hard to protect sliding glass doors. Requires three lithium button cell batteries (included). This unit has a disarm key to activate the unit instead of an on/off switch. This prevents the alarm being easily disarmed. the only way to disarm the unit is to insert the disarm key. 2 ½” x 1 ½” x ⅝”