Today's Blogging A-to-Z challenge post sits right in the middle of everything.
The tritone is the interval between the perfect 4th and the perfect 5th. Depending on which direction you're going, it's either an augmented 4th or a diminished 5th. And it's always going somewhere.
In the C major scale, the natural tritone is between F and B (where it's an augmented 4th) or B and F (where it's a diminished 5th). B, remember, is the leading tone in the key of C, so it really, really wants to resolve to C. The F (subdominant) wants to resolve to E (the mediant). Therefore, if you have an augmented 4th, it wants to resolve to a minor 6th (which is also the tonic chord in first inverstion):
And if it's a diminished 5th, it resolves to the major 3rd:
Remember ethnic 6th chords? Tritones all the way down. Even in a basic I-V7-I progression, the V7 has the tonic chord's tritone in it:
The tritone is between the first and second lines from the top. Just as in the very first example on this post.