Diabolus musica - The devil in music.

 Diabolus musica is also known as a tritone, an augmented 4th, or a flattened 5th.

Let's look at this interval happening between the notes E and Bb (A# is another name for the Bb note).

I've also named the notes C & G.

In the really old days this interval was called the devil in music because people didn't like the sound of it. It happens over three tones, which people called 'the holy trinity'. Looks like the devil had the holy trinity covered!

This interval became the best friend of Jazz and Blues musicians. In Jazz it's also called the #11 when it is placed above a chord. Personally, I find it to be the most exciting sound in Jazz.

In the above keyboard picture I outlined the notes for the chord C7 - C  E  G  Bb. Most Jazz musicians would probably agree that the E and Bb are the most colourful notes in this chord. When you're improvising over C7 it sounds really nice if you play these two notes. 

On the violin I often use these two notes as a double stopped (played on two strings) chord (C7 in this case).

It gives a great sound, AND sounds like the chord. The interesting thing is that there is a tritone between each note, so it doesn't matter which way around you place the notes.

E  * | *  * | *  Bb

Bb * | *  * | *  E

*  * represents two semitones = a tone (so does Bb  * or *  E)

I hope you found that interesting.

Have fun but don't use just root notes in your solo.