See: Headphone Jack (3.5mm) 2.5mm connectors are still common ... depending on the number of separate connector rings on the plug and jack. 2.5mm refers to the approximate diameter of the connector.

Dependent on the number of connector rings, headphones can also include a microphone. The current trend is for high-end smartphone flagships to exclude the 3.5mm jack, with headphone support being ...

It can support stereo and/or microphone, depending on the number of separate connector rings on the jack. Some phones offer only a 2.5 mm jack, which is a smaller variety of the same principle.