Pan law
The rule for how much a mixer turns a sound down when it's panned to the centre, so it doesn't seem louder there than at the edges.
A centred sound plays equally loud from both speakers, and two speakers add up in the room. Without compensation, the level jumps every time something moves toward the middle. The fix is to attenuate the centre: −3 dB keeps acoustic power constant, −6 dB keeps level constant when the channels are summed to mono, and −4.5 dB splits the difference.
None of them is right everywhere, which is why every DAW ships its own default and some let you pick per project. You usually notice only when a mix moves between programs and the balance is suddenly slightly off — with not a single parameter changed.