The tritone sound in the major scale!!!

Think of the major scale.  Pretty consonant.  However, we must remember that even the not so consonant tritone sound achieved by 3 whole steps in distance from the any one tone to another one, is also in there.  Use this to your advantage when composing.

Q:  So where does it happen?  

A:  From the 4th degree to the 7th degree (in C major this is from F to B).

A:  From the 7th degree to the 4th degree.

You can use this for compositional purposes to add dissonance and resolution when scoring visuals or simply composing melodies.