T = tip = send (generally)

R = ring = receive (generally)

S = sleeve = ground

I'm guessing this is what you mean by a barrel socket;

It's easy to tell which is Tip, Ring and Sleeve. Just start with the end you plug the jack into; the nearest lug is the sleeve, ring is in the middle and the tip is at the end. In the case in the above picture, from left to right, the lugs are: Sleeve, Ring, Tip.
Last edited by Mathamology at May 26, 2010,