There are many melodies that use fewer than seven notes, but I'm not going to cover them here. There is also a body of art music that deliberately uses more notes, sometimes going so far as to use all twelve notes on the piano the same number of times. A modal analysis of Schoenberg would be interesting but beyond the scope of this page.
Assume a palette of twelve distinct notes from which we can choose our seven-note scale. Most often these twelve notes are the 12-tone equal tempered scale used in Western music.
Now, take this same instrument and play a song that centers around A. It sounds completely different, more solemn and less cheerful. We call this sound "Minor". I conjecture that the reason the notes are named so the major scale starts with "C" and the minor scale starts with "A" is that the church composers considered the minor scale to be more important.
Given a seven-note instrument, we can obviously choose any note as the keynote. Tunes centering around each note sound different, and scales beginning with each note were given special names as "Modes".
("t" stands for Tone and "s" stands for Semitone) MODE Note Note Note Note Note Note Note Note Interval Aeolian A t B s C t D t E s F t G t A Locrian B s C t D t E s F t G t A t B Ionian C t D t E s F t G t A t B s C Dorian D t E s F e G t A t B s C t D Phrygian E s F t G t A t B s C t D t E Lydian F t G t A t B s C t D t E s F Myxolydian G t A t B s C t D t E s F t GThe names assigned to the modes come from regions of ancient Greece. The Church theoreticians thought that music in the various modes reflected the character of the regions.
A majority of Western music uses either the major scale (Ionian mode) or the minor scale (Aeolian mode). Many Celtic tunes are in Mixolydian and Aeolian mode, Phrygian has a flamenco sound to it, and Dorian is a popular Renaissance mode. The distinctive feeling of Myxolydian tunes is the seventh note, F instead of F# as it would be in G minor. Dorian has sixth note B instead of the Bb expected in D minor, and Phrygian has second note F instead of the F# expected in E minor. Celtic Aeolian music tends to use Am, G, and F chords where minor-scale music tends towards Am, Dm, and Em.
For the purposes of having names to talk about, I will call a tetrachord that goes up by a tone, a tone, and a semitone a "major tetrachord". Example: C, D, E, F is a major tetrachord and so is G, A, B, C. A tetrachord that goes up tone, semitone, tone is a "minor tetrachord", like A, B, C, D or D, E, F, G. One that goes up semitone, tone, tone like E, F, G, A will be called a "subminor tetrachord". Now we see that five of the Church modes break into tetrachords:
Mode First Tetrachord Second Tetrachord Ionian Major Major Mixolydian Major Minor Dorian Minor Minor Aeolian Minor Subminor Phrygian Subminor Subminor
By combining the three tetrachords, we get four other modes: minor-major, subminor-minor, major-subminor, and subminor-major. Minor-major is seen in classical analysis as the ascending melodic minor scale. The first three of these new modes can all be played by tuning C sharp but leaving F natural, giving A major-subminor, D minor-major, and E subminor-minor. To get a subminor-major scale you have to retune two notes: C# and D# gets us E subminor-major.
Just because a mode exists doesn't make it sound good. The only one of the four non-Church modes I really like is minor-major, which lets me play tunes where the chord on the keynote is D minor but the chords on the fourth and fifth notes are G major and A major.
To play in A harmonic minor, you would tune your G to G#. This is an interesting tuning. If you start your scale at E, you get a gapped tetrachord E-F-G#-A with a subminor tetrachord on top. This scale is sometimes called "Jewish minor" because a lot of Jewish tunes (think Havah Nagilah) use it.
Much Eastern European music uses a scale of two gapped tetrachords. It is especially characteristic of Rom music.
With four tetrachords to choose from, we can get sixteen different modes. Not all of them will sound good to you.
A gapped upper tetrachord gives a "leading tone" a semitone below the last note. You can get even more urgent and have a second leading tone a semitone below that one. Assuming that the scale does contain a fifth and the upper tetrachord starts on it, that gives a tetrachord of a gap and two semitones. Try that one over a Ukrainian tetrachord.