The analog socket used to plug in a headset, earphones or headphones. The 2.5 and 3.5mm jacks have been used on cellphones and smartphones, while stereo and home theater equipment generally have a 6.3mm socket.
Common on mobile phones for decades, the 3.5mm headphone jack is vanishing. For example, there is none starting with the iPhone 7 in 2016 and Samsung's Galaxy S20 in 2020. An adapter for a wired headset or a Bluetooth wireless headset must be used instead. See mini-phone connector
, phone connector
and Lightning connector
The 2.5 and 3.5mm jacks have been used with mobile devices, while the 6.3mm connectors (bottom) are found on non-portable audio equipment.