Headphone jacks come in many varieties. The most common now is the 1/8th inch headphone jack. It is a small tip that fits most standard walkmans and cd players. Another type of jack is the 1/4 inch jack. It is used primarily in home stereo systems, and it's headphones are generally much larger. It is possible to convert from 1/4 to 1/8, just run over to radio shack and you can buy a converter for about $2.50. There are other types of jacks, including one that has 2 1/8th inch mono plugs, but they are used so infrequently thatI have no real use for them.