Many DJs mix in key, this is a technique called Harmonic Mixing. Tracks have a main key. Mixing in key allows you go from one track from another in a similar key.
A musical scale is a series of notes with unique pitches. The C scale goes from C to G, then A back to C.
Understanding the camelot wheel
The camelot wheel is a collection of keys or chords displayed visually in a wheel. Unfortunately we don't have the rights to the image to show you it, but showing you the graphic probably won't be useful anyway.
The camelot wheel is basically two clocks. One is inside another.
The clocks have the same numbers as a regular wall clock, the numbers go from 1 to 12.
The individual keys on the wheel items are made up of several notes. Items near each other would be made up of a few of the same notes.
One has each number ending with an A. The other clock is inside this and has numbers that end with a B.
3A would connect to 2A, 3B and 4A. 12B is most compatible with 12A, 11A and 1A.
Some DJs mix diagonally, too. For example, they go from 4A to 5B, or 12A to 1B.
Some track combinations may sound fine when jumping between two or three numbers. For example, 4A to 6B, but you'd probably want to at least trial the track combination first.
Each letter number combination represents a key. However you don't have to remember the keys you just have to pay attention to the representations of the notes.
There's an alternative system called OpenKeys. This is just a system which arranges the keys differently.
In most DJ software you can select which system you use. Most people use the camelot system.
A seven point jump in number is regarded as an energy boost. A simple example is going from 4A to 11A. If your room suddenly fills up, then you may want to do this.
Similarly, you can lower the energy by moving the numbers down a few. For example you could go from 11A to 9B.
Alternatively, you may want to do something that I haven't specified just to mix things up.
You'd probably want to use the key lock feature of your DJ software.
Changing the tempo (the speed of the track) can change its key. To prevent the key changing you can just enable the key lock function in your DJ software.
Even the best software can only detect the correct key 85% of the time.
MixedInKey.com is the most accurate software. Most DJ software has key detection functions, but it's a bit less accurate.
Some DJs like the "advanced chart" that the MixedInKey.com software provides, because it gives them more options. You'll need to purchase Mixed In Key separately to your DJ software.
Be mindful that some DJs think that some of the track combination system sound terrible and some combinations may work which the software wouldn't suggest.
AutoKeyChain.com a website that allows you to upload tracks and it will detect the root key of tracks.
There's a software package called tuneXplorer by Abyssmedia.com is another external program for key detection.
ibrahimshaath.co.uk has a free software package called KeyFinder. This is primarily for iOS. There's a Windows version, but it's not supported or updated.
Most external solutions except AutoKeyChain modify your audio files to insert the information about the key. This allows your DJ software to use the information.
The B.P.M. of the track is usually detected by these software packages, too.
Consistency is essential
It's essential that you analyze the tracks with the same software; otherwise the combinations may not be as accurate.
Test the track combinations
As noted in the "software accuracy" some track combinations that the software suggestions may sound bad. You'd probably want to test the combinations before putting them in a live mix.
Changing track keys
The newer DJ software has a feature to allow you to change the key of a track. Adjusting the key of a track up or down by 1 or two keys would allow you to virtually create a smooth mix regardless of the tracks.
If you do requests and don't wish to jump keys, then you'd be better off using DJ software to change the key of the tracks to make the keys more compatible with the track that you're playing.
You can also play other songs in between to better transition to the key of the requests.
Some DJs who take requests don't worry about harmonically mixing.
Harmonic mixing with vinyl
Harmonic mixing generally wasn't done with vinyl. It can be done if you want to do it.
You would need to make a note of the track keys, and then find the appropriate record. You will need to scan a copy of the tracks to get the keys of the tracks. Alternatively the keys might be available online.
Why you may not want to mix in key
Some DJs may find it better just to trust your own instinct on what works.
Relying on software to pick compatible songs is objectionable to some DJs.
Some DJs may find the system limiting.
Some devices have key sync which enables you to quickly find tracks in near keys. The problem with this is it doesn't give all the options so I recommend that you don't use this.
Prominent DJs who do harmonic mixing
Many prominent DJs do harmonic mixing including Paul Oakenfold, Sasha, Armin van Buuren, and Markus Schulz.
Thanks for reading
You may want read our guide on how to DJ and mix music.