Tritones

a tritone is the middle note of an octave. there are 11 half steps before you reach the octave, so the interval between the root and the 6th half step. that's in either direction, up or down. it's also known as the augmented 4th or diminished 5th. they're the same note.

g and c# are 6 half steps apart. c# is halfway between any two g's, and g is halfway between any two c#'s. c and f# is another tritone. e and b flat is another... etc.

it's a real dissonant sound, and you rarely find it in anything except jazz chords. hendrix used it in purple haze, in the e7/#9 chord, but you see it much more in a vii7dim chord.

Gotcha

Ddim7 (x 5 6 4 6 x) = D F Ab B. You would have two tritone intevals: D- Ab and B-F, or vice versa.

Correct?

Tritone = 3 (tri) whole steps...augmented 4th or flatted 5th, depending on which note you start from.

G-C# = +4
C#-G = b5


Tritone occurs naturually in major scale on 4th and 7th scale degrees, so your example could be considered key of D (G is 4th step and C# is 7th step in D). Tritone is dissonant ("devil's interval") and wants to resolve (at least on classical theory), sometimes "outwards":

C# -> D
G -> F#


which would be A7 to D (V-I)

could also resolve "inwards":

C#(Db) -> C
G -> Ab


which would be Eb7 to Ab (V-I)

In this case, Db is 4th step and G is 7th step of key of Ab...

Note - D and Ab is a tritone interval as well...

Also, in jazz, the same tritone is shared in the dominant chord and it's substitute, so in above example, you could use an Eb7 in place (substitute) of an A7 in key of D.

Noteboat covered that in his latest lesson (item 10 at the bottom of the page):

https://www.guitarnoise.com/lessons/chord-substitution/

Thanks :)

Just as a bit of trivia, the begining bit of the Simpsons theme is a tri tone. three tones between each "The", "Simp-" and "Sons".

Fascinating stuff.

Peter