In music, a difference of three whole tones between two notes can get all of the above names. This might have you asking one of the following three questions:
  1. Good point, Footprints, why are there so many names for one interval, and what is the difference between them?
  2. What the hell are you talking about?
  3. Who cares?
If you asked question number 3, then go read something else, maybe something like Grandma Got Run Over By a Reindeer.
If you asked question 1, skip the introduction and go straight to the explanation.
If you asked question 2 exclusively (i.e. without adding question 3), read on:


The chromatic scale has twelve notes, and they are all separated by half a tone or a semitone. And from this, we can build all the other intervals. The scale is:

C (B#), C# (Db), D, D# (Eb), E (Fb), F (E#), F# (Gb), G, G# (Ab), A, A# (Bb), B (Cb)

In brackets are the enharmonic names of the notes. So between C and C# there is half a tone. (Between C and Db there is the same half tone). Between C and D there is a whole tone. Between C and E there are two whole tones. Between C and F# (or Gb) there are three whole tones. Also, between A and Eb there are three whole tones, as it is in fact a circle (after B comes C). Three tones are called a tritone. This interval is sometimes referred to as an augmented fourth, a diminished fifth, a flat fifth (b5) or a sharp eleventh.


Tritone means three tones. So when there is a difference of three tones between two notes, this is a tritone. This terms is used when the interval is out of musical context. For example, if in ear training, if I play C and Gb and ask you what interval I played, you would (should) answer tritone (this is true, as you have no idea whether I'm playing C and Gb, C and F#. (Or, for that matter, B# and F#)). As we shall see, there is difference between them when they are in a musical context.
Diminished fifth and augmented fourth
If the tritone is in context, we shall see it is either an augmented fourth or a diminshed fifth. A perfect fourth is a distance of four notes on a major scale. For example F is a perfect fourth above C. Using a quick calculation, we find that F is in fact two and a half tones above C. A perfect fifth is a distance of five notes along a major scale. So G is a perfect fifth above C. By the same calculation, G is three and a half tones above C. Augmented, in music, means adding a half tone, and diminished means lowering a half tone. So as you can see, adding a half tone to a perfect fourth (making an augmented fourth), we'd get three tones, and lowering a half tone from a perfect fifth we get a diminished fifth, also three tones. Let's apply this to the scale. An augmented fourth from C is F#. A dimished fifth from C is Gb. (# stands for + half a tone, b for - half a tone). Now F# and Gb are enharmonic (they sound the same). But they are not technically the same.

Gb is NOT an augmented fourth from C. It is a fifth (G) that has been diminished. I'll give some more (more difficult examples) - a perfect fourth from B is E. An augmented fourth from B is not F. F is a diminished fifth, as F# is the fifth. An augmented fourth would be E#. The fifth of Gb is Db. What would it's diminished fifth be? Not C, but Dbb. Simple, see?

#11 and b5
When dealing with intervals, we use the terms "tritone", "diminished fifth" and "augmented fourth". However, when dealing with chords and scales we use the terms b5 (flat fifth) and #11 (sharp eleventh).

For example, let's take a look at the C mixolydian scale: C, D, E, F, G, A, Bb - C is the first note of the scale, D is the second, E the third, etc. But chords are built using thirds, so take C, add the third, E, and the next third, G. We get C, E, G - C major. Adding the next third we get C,E,G,Bb - a C7 chord. Add the next third - C,E,G,Bb,D - C7,9 (often called C9). (Notice d is the ninth. It is also the second, but it was added after the seventh, so it is the ninth). Continue if you wish - it's easy. F is the eleventh of the scale. What if we raise F half a tone? We get a sharp 11th (#11). This would give us: C,D,E,F#,G,A,Bb. This gives us a mixolydian #11 scale. (This is actually known as the lydian b7 scale, but that's just a name). In it, we still find the major triad (CEG) and the seventh chord (C7), but the fourth (eleventh) is raised half a tone from the mixolydian.
What if we take C mixolydian and lower the fifth? The fifth is a G, so we get a Gb. We get C,D,E,F,Gb,A,Bb. As you can see, this is a totally different scale. It can be called mixolydian b5. It is completely different from the previous (lydian b7), as you can see. In fact, this scale is not used at all in jazz or classical music, whereas the lydian b7 is constantly used (in jazz at least).

When talking about chords, a C7b5 chord has C, E, Gb, and Bb. A C7#11 has C, E, G, Bb and F#. Play these on the keyboard. They are very different, and are used in different contexts. Actually, we don't often get a b5 in a major chord. It is most commonly found in minor chords (thus making them diminished, or half diminished. This is actually where the diminished comes from - it's a diminished fifth) Also, when improvising over them, you would use a lydian b7 over the C7#11, but (most probably) a whole tone scale over the C7b5.

So as you can see, this little semantic difference is not so little after all!