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

Just for added info they’re called “Tip and Ring”, “TRS” (Tip, Ring, Sleeve), and “TRRS” (Tip, Ring, Ring, Sleeve) connectors for 2, 3, and 4 conductor connectors respectively.