Mixing Major to Minor Tracks

So I’ve been learning about the concept of Major and Minor key tracks. Basically, tracks played in major key typically sound happy and songs played in minor key typically sound sad.

The issue with this is that mixing major to minor tracks can be difficult since they would normally clash. They wouldn’t be in harmony, well, without using some tricks.

Typically, in DJ software, tracks in major key are marked with a ‘B’ and tracks in minor key are marked with an ‘A’. Now I don’t know the theory behind it, but each key signature is assigned a numeric value from 1 to 12 as well.

Most DJ tracks are in minor, so if you ever find yourself wanting to mix in a minor track while in a major track in say 7B, then you use a quick rule of subtract 3. So you would be mixing in a minor track in 4A.