What are Scales?
The Western music is based on twelve semitones. This system is called a chromatic scale by opposition to the diatonic scale which consists of five full-tones and two semitones placed in a specific order according to the chosen modal tonality system. (meanly major and minor). The term diatonic comes from ancient Greek music theory and means "through full tones." Ancient Greek music tuned its scales using an interval of a perfect fourth (P4) called tetrachord. A diatonic tetrachord was one that was tuned with two whole tones on the top, and the remainder left on the bottom (roughly a semitone). (See Modes and Pentachord). These tunings in ancient Greece were contrasted with chromatic methods of tuning the tetrachord, which generally involved intervals smaller than whole tones and therefore often resulted in some consecutive semitone intervals as we find in our modern chromatic scale. (See Introduction)
Figures a, b, c show diatonic C scales with their respective five full-tone and two semitones. Also, note the relative similarity between a major and a melodic minor scale. (Fig. a), (fig. c) No wonder that melodic minor was chosen for vocal. Also, note that Natural minor has no leading tone. (Fig. b)
Each note of a scale has a particular name called, Scale degrees name. We will use them to single out notes function in chords.
The construction of the Major scale
Let's have a look at what a Tetrachord is. One can easily notice that the lower and upper Tetrachord is exactly the same, "One tone, one tone and a semitone". The semitone between the 7th degree (B) and the "upper tonic VIII, C" is called Leading Tone, since B is greatly "attracted" by C. The same with the IV degree F, he wants to go downwards to E. This downward and upward motion is "orchestrated" by the Tritone placed on the IV and VII degree. Also to mention that the upper tetrachord begins with G which is called Dominant. The term Dominant is associated with its position in a key. The fifth note of the scale and the chord built on that note give a perfect 5th. (P5). See the table "Type of chords"
This time one has a G7 scale with a G7 Chords called Dominant 7th chord. Although the notes, from the lower and upper Tetrachord one, are placed the same, now one has the Tritone that has moved one step down. What does that mean? We have seen that B wants to reach C and at the same time F wants to do the same but downwards, one has then the reconstitution of the C chord (Tonic) with the notes C and E adding then the fifth degree G.
Finally, we have come to the final step. To examine the function of the Dominante seventh Chord. The function of the dominante 7 seventh chord is to resolve to its Tonic (here G7 to C). This motion is called perfect cadence. The Fifth degree resolving to the first degree by a downward and upwards motion (F to E) and (B to C). The 7th up to the 1st and the 4th down to the 3rd.
Relative Minor/Major Scale.
What is a relative minor scale? The picture below shows the notes in a C major scale that continue until they reach a twelve intervals scale. The first seven notes belong to the C major scale (blue section). The other seven notes (in red), starting from degree Vl in C major, form the A minor natural scale and thus become the relative minor of C major scale. (some sort of complementary scale to C major). Please note that C major and A natural minor have no alterations. (no sharps, no flats). On the other hands, the natural minor scale starting on the sixth degree of C major scale represents the Aeolian mode.
We have also to mention the relative Major of a minor scale. Here, C natural minor and its relative Major, (E♭M7). See also where the Tritone is placed in the natural minor scale.
At this point, it is essential to understand the relationship between tones and semitones in tonality. Effectively, the notes that form a scale have not the same "power". For instance, (fig l.) shows the notes C, E, and G in the C major scale which are commonly called "rest or stable notes" and thus do not to resolve. Whereas, (fig ll) shows notes that may be defined as "unstable or active notes" since they are attracted by either the preceding or the following note. D is attracted by either C or E and A is by G. But the most unstable notes are of course F and B namely the Tritone (T). In the C scale, the B note is called leading tone.