Gaming What is the port next to the headphone-in used for?

[Brad01]

http://www.8-bitcentral.com/images/nintendo/ds/headphone.jpg

What is the thing to the left of the headphone jack? I think it's present from the DS Phat all the way to the first 3DS. I've been seeing these things for over a decade now and have never figured out why it's there.

 

[Mnecraft368]

http://www.8-bitcentral.com/images/nintendo/ds/headphone.jpg

What is the thing to the left of the headphone jack? I think it's present from the DS Phat all the way to the first 3DS. I've been seeing these things for over a decade now and have never figured out why it's there.

I +1 this.
I think I found out once but I cant remember.

 

[VinsCool]

Headphones+Microphone combo IIRC

 

[CMDreamer]

Those are the connection ports for the microphone and the headphones as an all in one device. The icon even resembles them.

 

[ov3rkill]

Mic for voice/party chat. This thing beats the Switch in terms of voice chat convenience.