Musical key is one of the most important considerations in every remix.
At the very least, you need to make sure that all the elements you add are in key with the parts you’ve brought over from the original track.
Learning some music theory can come in very handy if you’re going above and beyond the usual remit. Playing with the key signature of the original song and stamping a new one on your own version is an audacious thing to do, but if you don’t do it right, your remix will fail miserably.
If you’re going to mess with the key of the track, you’ve got two options...
1. Brute force
The first is to manipulate pitches, transpose samples and force it into a completely new scale. Using modern tools like Melodyne, Auto-Tune or even quite basic DAW or sampler timestretching features, it’s not exactly hard to make a remix conform to your design by brute force.
Transposing elements by several semitones or even whole octaves is certainly one way to make a statement.
2. Relative keys
The second, arguably more elegant option is to use a relative key. This is a more clever but quite technical way to do things, and it makes use of the original elements as they are, without the need to tweak any pitches.
The relative minor key of C major, for example, is A minor. These two scales share the same notes, but the scales just start at different points. C major is C D E F G A B C, and A minor is A B C D E F G A. Thus, anything from C major will also sound good in A minor.
You can take this concept even further. Analyse what notes the elements you’re bringing over from the original song are made up of. Your selections may contain all seven notes from the original key, or less.
If they contain, say, five notes in total, your key can be any one that contains those five notes. Use a scale finder to enter the notes you have at your disposal and identify the keys they can fit into. Search only ‘normal’ scales for the best and most logical results.