Based on how the microphone and earpiece connect to the walkie talkie connector, users can select between a 1-wire, a 2-wire, or a 3-wire surveillance kit earpieces.
When the two-way radio accessory has one wire leaving the radio connector and if the ear piece portion is connected to the microphone in any way, matter, or form then you have a 1-wire radio earpiece.
The 2-wire earpiece portion and the microphone portion of the radio accessory each have their own wire as they leave the radio connector.
The 3-wire surveillance kit has three wires coming out of the radio connector. One wire for the microphone, another wire for the ear piece, and a third wire for a PTT button. The user can wear the PTT button in many locations that can be discretely pressed to make a radio call.