4-conductor 2.5mm TRS pinout?

on a 4-conductor 2.5mm TRS male jack, used in a phone headset, which conductors lead to which parts? the plug i'm curious about looks like this: a href="http://www.infinitecables.com/images/connector_sm/3.5mm-4c.jpg" rel="nofollow"http://www.infinitecables.com/images/con…/a