The #iv chord…
A minor triad (or minor seventh chord if you choose to expand the harmony) built on the #4 scale degree… which is a tritone away from the tonic.
One use for it is as a loose chromatic approach from the IV to the V. For example, take the following harmonic progression:
Now, let’s insert the #iv chord in between the IV and V of the progression. Notice the chromatic lines and how it makes this interesting chord less “out-of-place” with the key:
It does soften the blow of the cadence because the ear is trying to figure out what key we are in, but it can be used for coloristic effect.
On the same idea of chromatics, we can substitute the IV with a #iv chord in a vi-IV-V-I progression with the use of chromatic voice-leading:
Play around with it and see how it sounds!
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