Why does a headphone plug have a different number of lines?

Question

Whenever I see a headphone plug, it has lines on it, those being from a number of 1 to 3. Why is this?

In: 20

Answer 1

Lines are there to isolate different conductors

1 line – 2 conductors = 1 channel (one ear + ground)

2 lines – 3 conductors = 2 channels (left ear, right ear + ground)

3 lines – 4 conductors = 2 channels + microphone (2 ears + microphone + ground)