This slide discusses the two types of signal relays. First is a non-latching relay. It is made with a single coil, and it works by activating the contacts, or the switch, whenever there is current in the coil. Think of it as a momentary push button. The other type of signal relay is a latching relay. With a latching relay, the switch or the contacts are activated even when the current is not flowing. If it is a single coil style, the polarity of the voltage will determine whether the relay latches or unlatches. If it is a dual coil latching relay, which is also very popular, one coil will activate the relay, while another will reset the relay, or turn off the switch. Non-latching and latching relays are not interchangeable.