Diatonic-Scales
Diatonic scales (the so-called "Major scales") are probably the most frequently used scales in music. "Diatonic" roughly means "through tones" in Greek. A "tone" here means two half-steps, or two semitones. Therefore, by saying it's "diatonic", you're saying the scale goes through "tones" as well semitones.
Let's spell the C diatonic scale. It's C, D, E, F, G, A, B - exactly what you get when you go up the white keys on piano. The interval between C and D is a tone (two semitones), the interval between D and E is also a tone, and the one between E and F is a semitone. So, if you count from C to F, the intervals are tone-tone-semitone. The intervals between F and G is a tone, and so are the ones between G, A and B. Then the one between B and C is a semitone. Therefore, if we go up from C to B, we'll have the tone-tone-semitone-tone-tone-tone-semitone order. The triads to be formed out of this diatonic scale are C, Dm, Em, F, G, Am and Bdim. The 7th chords are C?7, Dm7, Em7, F?7, G7, Am7 and BO. Out of these chords, F and F?7 are the IV chords, and G and G7 are the V chords.
That's the fixed proportion of diatonic scales, and it's constant whatever note the scale is rooted on. For example, if you spell a diatonic scale rooted on D, you'll start with D and pick up the notes conforming to the "tone-tone-semitone.." rule. Namely, they are D, E, F#, G, A, B, C#. The F in the C diatonic scale became F# to widen the interval between that and E, and, in turn, the interval between that and G narrowed to become a semitone. The same goes for the C-to-C# change. The triads in this diatonic scale are D, Em, F#m, G, A, Bm and C#dim, while the 7th chords are D?7, Em7, F#m7, G?7, A7, Bm7 and C#O. The G and G?7 are the IV, while A and A7 are the V.
By the way, diatonic scales are tightly related to "keys". When you say the key of a certain tune is C major, you mean that the primarily used diatonic scale in the tune is the C diatonic. There are 12 notes in an octave, which allows 12 different diatonic scales to exist. Mind you, this doesn't mean there are only 12 possible keys in music. In fact, there are 24. Why? That's because while a diatonic scale can only be rooted on one of the 12 notes, it can choose where to place its center out of two choices (*). One choice gives you a major key, and the other gives you a minor key. (The examples shown in this entry are for the major-key option, I'll talk about the minor keys in another entry. The notes in blue in the keyboard images are the center for minor keys, by the way.) J.S. Bach's The Well-tempered Clavier consists of 24 prelude-fugue sets, which is the result of having each set rooted on the 12 notes and both in major and minor keys.
Let's see the Eb diatonic scale. See how the "tone-tone-semitone" order is still the same? This, by the way, is probably the first diatonic scale out of the 12 that alto sax players would learn. The horn is tuned to the Eb key by default. Alto clarinet too. The triads in this diatonic scale are Eb, Fm, Gm, Ab, Bb, Cm and Ddim, while the 7th chords are Eb?7, Fm7, Gm7, Ab?7, Bb7, Cm7 and DO. The Ab and Ab?7 are the IV, while Bb and Bb7 are the V.
How about the E diatonic scale? This is the favorite diatonic for guitarists, since their axe's bottom and top strings are tuned to E. (It's a nightmare scale for the alto sax players to play, though.) It's not a very friendly thing for pianos either, as you can see there are four black keys involved, which can disrupt your finger movement considerably.
(*) I'm excluding the possibilities of so-called "modal music" here, to make explanation easier.
Scales: Diatonic Scales関連ページ
- Guitar: Diatonic Scale Positions
- Electronic Musician
- Scales-Minor-Scales
- Electronic Musician