A system for detecting the position of at least one moveable element of a window or door, the system comprising: at least one sensor for sensing a magnetic field, the at least one sensor being configured such that the magnetic field sensed changes as said moveable element moves; and a processor configured to receive output signals associated with the sensed magnetic field and to determine the position of said moveable element; wherein the system is configured to operate in a calibration mode and a normal mode, wherein in the calibration mode the system is configured to register at least an output value from said sensor when said moveable element is at a first predetermined position as a corresponding first reference value and wherein in the normal mode the processor means is configured to use the first reference value in determining the position of the at least one moveable element.