Stereo

Stereo carries two channels. Feeding them slightly different signals lets a listener place sounds across a width between the speakers, which is what music mixes use and what makes a stereo recording of a room sound like a room.

Dialogue is where people come unstuck. A voice recorded on one microphone belongs in the centre, meaning the identical signal in both channels. Record a single microphone into one input, leave it alone, and half your audience hears you in the left ear only.