I agree with the last posting.
Sometimes the sleeve connection of a mono socket, is a spring finger that makes contact with the ring of a stereo jack.
If you only have a stereo jack and want to use it as a mono. Wire the sleeve and ring together to be on the safe side.
For stereo jacks, convention is that the tip is Left.
3.5mm and 2.5mm Jacks with four connections are now becoming more common on A/V equipment, laptops and some amateur radio gear. Be careful with these. As the position of the ground connection varies considerably, and it's never where you think it should be.
Martin - G8JNJwww.g8jnj.webs.com