Analog trunk circuits connect automated systems, such as a private branch exchange (PBX) and the network such as a central office (CO). The most common form of analog trunking is the E&M interface. E&M Signaling is commonly referred to as "ear & mouth" or "recEive and transMit", but its origin comes from the term earth and magnet. Earth represents electrical ground and magnet represents the electromagnet used to generate tone.