This stereo mini female to 2 1/4' mono male Y-cable can be used to split a stereo mini 3.5mm signal to two 1/4' mono connections, or to combine a left and right 1/4' signal into one mini stereo female jack.
This is a TS or mono plug (note the single stripe on the connector) This provides single channel analog audio.
TRS (Stereo) Plug (Male)
This is a TRS or stereo plug (note the two stripes on the connector) This provides a stereo or 2 channel analog audio. Commonly found on headphones
TRRS Plug (Male) 4 Conductor
This is a TRRS 4 conductor plug (note the 3 stripes on the connector) This can provide 2 channel audio plus a microhone. Also capable of 2 channel audio and a video feed. This is commonly found on iPhone headsets and portable DVD players.
