When people familiar with metal detector coils hear certain terms, they think certain things. Here are the three classic coil designs, illustrated in many articles in books, magazines, and the internet. Mono and DD coils are common, concentric much rarer with modern VLF and PI detectors. A classic mono transmits and receives alternatively with a single coil winding, and so is only used on pulse induction detectors. DD and concentric separate the transmit and receive into two separate windings, a