Just a quick note... Mixed in Key is no longer necessary as both Pioneer and Serato's DJ apps include this functionality in the base, free versions.
Also, GP might want to look up the 'camelot system', which relabels keys in a simple numeric system which makes it a lot easier to determine which keys are compatible without memorizing the entire circle of fifths. Every DJ I have met or played with that keymatches labels their tracks in this fashion.
> Just a quick note... Mixed in Key is no longer necessary as both Pioneer and Serato's DJ apps include this functionality in the base, free versions.
At least in Rekordbox (Pioneer), Mixed in Key is the only way to display both the actual key and the Camelot notation. This is useful for knowing which songs can be easily brought in sync using key/pitch adjustments in software. For example, 9A and 3A are not necessarily easy to mix together, but if you know that 9A is Em and 3A is B♭m, it's only a single half-step adjustment in Rekordbox to make the latter 10A, which is usually easy to mix into 9A.
Mixed in Key and the other tools that they provide can also do more than just identifying the key of a song. I wouldn't say it's essential for a beginner, but it justifies its price for someone who uses it professionally or even semi-professionally.
Actually you hit on one of the lesser-known rules of Camelot system, that +/-7 (aka perfect 5th[edit, this is not right see below]) is mixable, though at a higher danger of clashing. Same with +/-3, at even higher risk. When I read about this the lesson was to do your transitions quickly.
This applies to track mixing as a DJ, not sure how mash-up mixes change things.
https://en.m.wikipedia.org/wiki/Circle_of_fifths
https://en.m.wikipedia.org/wiki/Mixed_In_Key
and by adding beatmatching we get very nice mixes or transitions.
after a while you won’t need either of these as things will start working out by themselves.