This is a security alarm for windows and doors. When the installed window or door opens a certain amount, a loud 95dB alarm sounds to deter intruders. Double-sided tape is attached to the back, allowing for easy installation on window frames, etc. I accidentally ordered the wrong quantity and have been storing these, unused and brand new. I have no plans to use them, so I'm selling them as a set of 3. Size: * Main unit: Approximately 9cm x 3cm x 1.9cm * Magnet: Approximately 5cm x 1.4cm x 1.2cm *Requires 2 AAA batteries (sold separately).