The Apple speaker minijack is a stereo 2.5-mm miniature jack. It has a smaller diameter than the headphone jack so that the user cannot inadvertently plug headphones into it.
WARNINGSome types of headphones and other audio devices have a 2.5-mm plug. The user should be warned not to plug such devices into the Apple speaker minijack. Doing so could cause damage to the devices.
The Apple Speakers include an internal ROM that enables the computer to identify the speakers. Speakers other than the Apple Speakers should not be connected to the Apple Speaker jack.