Contact sensors have been designed to fit a window or door frame to provide a meth9od of activation should the door or window be opened when the system is armed. The ago between the sensor and the magnet housing must be no more than 25mm or this will be detected as an activation.
The diagrams below show the sensors attached to a door and window frame:
Contact Sensor mounted to the door frame
Contact sensor mounted to the window frame (vertically and horizontally)
For all mounting points, you should place the magnet section of the sensor (the thinner of the two parts) to the moving part of the door or window. The sensor continuing the battery should be mounted on the fixed frame. In all cases remember the gap between the magnet and the sensor should be no more than 25mm when the door or window is closed to ensure it can be armed correctly.