One of my most favourite and easy to use reharmonisation tools that I use all the time is the principle of the tritone substitution.
In it’s most basic form we take any dominant 7th chord and replace it with a another dominant chord that is a tritone away from the original chord.
For example C7 can be replaced with F#7. F# is a tritone or 3 tones (whole steps) away from C.
The reason why the tritone substitution works so well is that the new chord and the original chord contain the same guide tones. C7 has E and Bb (3 and 7) as the guide tones and F#7 has A# and E (3 and 7). Of course in the real world A# and Bb are exactly the same notes and the 3rd and the 7th have swapped.
Check out our video below and try the practice suggestion below.
Practice
Go round the cycle of 4ths playing 2 5 1’s and replace the 5 chord (dominant 7) with the tritone substitution in each chord. Notice that the left hand moves down chromatically.
Later I will show you how we extend and think a little deeper on how we can use this principle with and without other principles of reharmonisation. Have fun!