The tritone
Let’s take a closer look at the piano keyboard with the given intervals.
One key is not identified by an interval – the F sharp/G flat. Why not? Now, above and below there is a pure interval, namely the pure fourth, C - F, and the pure fifth, C - G. And pure intervals cannot be major or minor, so how do we describe the missing interval?
Well if we count off the tone steps, there are 3 whole tones from C to F sharp. If we continue to count upwards, we again get three whole tones from F sharp to C. The F sharp/G flat note splits the octave precisely in the middle.
Because to the distance of three whole tones, this interval is given its own name, the tritone (tri – Greek for three).
The tritone is a very special interval, and it is also very important. Isolated on its own the tritone sounds pretty weird – you might even say very “tense”. And it is exactly this that makes it so special. In the dominant seventh chord (see “4-note chords”) there is a tritone between third and seventh. This tension-filled interval, however, sounds very logical in this chord, and justifies the breakup tendency of the dominant seventh chord in the major 7 chord.
Admittedly we're getting a little advanced here now already. For the moment let’s just leave it at making sure you know that a tritone is an important interval, and in the C major example it is written above the C – F sharp or C – G flat notes.
