The "jack" is without a doubt the most commonly used connector for small-scale audio equipment. Jacks are normally divided into three different types, based on their diameter:
- 2.5 mm jack: The smallest jack;
- 3.5 mm jack: The traditional jack, which corresponds to a headphone jack;
- 6.35 mm jack: The jack used for semi-professional sound systems, in order to connect speakers, amplifiers, or microphones.
There are two versions of each of these jacks:
- Mono jacks, for sending monophonic sound. This kind of jack has two contacts: a reference, found on the body of the cord, and the signal on the tip.
- Stereo jacks, for sending stereophonic sound. This kind of jack has three contacts: The same two as its mono counterpart, as well as an additional ring for sending another audio channel.
In computer sound cards, the plugs for jacks are generally color-coded so users can easily tell which type of audio device each one connects to, and whether they are audio inputs or outputs.