IAN LANG ELECTRONICS
A stereo jack plug has three conducting elements, one for each channel and one for return that is common to both. Looking from 1 to 4 we see:
1. The tip, which is the left-hand channel
2. The ring, which is the right-hand channel
3. The sleeve, which is ground (the common return)
4, the casing, under which the wires are soldered to contacts (or sometimes screwed into contacts).
Because of the above appelations, these connectors are often known as TRS types (Tip Ring Sleeves) and commonly they are called phono plugs.
Stereo plugs come in a number of sizes and are suited to the devices they are designed to work with.
Common sizes are 6.35 mm which equates to 1/4 inch for larger hi-fi headphones, electric guitars etc. and which was originally used in manual telephone exchanges. Other sizes to be found are 3.5 mm and 2.5mm which are 1/8th and 3/32nd of an inch respectively. 3.5 are used on mobile phone and computer audio devices whereas 2.5 are used on micro devices.