Size:NC+NO 2Sets Switch Type: NO/NC, NC means Normally Closed:When the magnet together, the circuit is conductive( light on ); when the magnet separate, the circuit is not conductive ( light off ). NO means Normally Open:when the magnet separate, the circuit is not conductive( light on );When the magnet together, the circuit is conductive ( light off ). The reed switch is an electrical switch operated by an applied magnetic field. Designed for use with alarm systems. Great for use in family and factory window, door. etc. Screw Mounted for easily install.