A scale is a sequence of pitches, used to organize the melodic and harmonic structure of a piece of music. Each scale tend to give a certain underlying flavor to a piece of music, and often have emotional or cultural connotations.
The first note of a scale is the root, or the tonic. Any scale can be transposed to a different root note and stay the same kind of scale. Most scales are associated with a certain key, and a modulation usually involves a change of scale as well as a change of key. A mode is a scale that has the same pitches as another scale but starts on a different note. (Sort of. Unless you're talking about Gregorian chant in which case it means something related but different.)
The most common scales in Western music are the major scale and the minor scale. The C major scale is the notes C D E F G A B C and generally sounds positive. The A minor is A B C D E F G A and generally sounds sad. The piano keyboard is organized around these scales, which is why you can play C major and A minor using only white keys. The more different a scale is from a major or minor scale, the more ethnic or foreign it will sound to Western listeners.
- A chromatic scale is made of every one of the 12 notes, e.g. C C# D D# E F F# G G# A A# B C. This is the sound of someone running their finger up or down a keyboard. All other scales are a subset of this (well, in Western music).
- Pentatonic scales are scales with five notes. There are many different scales all over the world, but in Western music this usually means one of these following scales. They sound folky or Asian (as traditional East Asian music frequently used pentatonic scales) and are easy to sing.
- Major Pentatonic can be played using only the black keys on a keyboard, starting on F#.
- Minor Pentatonic is a mode of Major Pentatonic. It can be played using only black keys too, starting on D#. Example: Gershwin's Summertime (except for one note).
- "Blues scale" can mean a few different things, but usually it refers to a Minor Pentatonic with an added sharp 4th/flat 5th. A blues scale on D# would be D#-F#-G#-A-A#-C#. Think of pretty much any Blues Rock guitar solo.
- Overtone Scale has to be the oldest in the book. It occurs on any brass instrument when not using the valves. The overtones follow a distinct pattern, and because of constantly diminishing intervals, they become a scale from number 8 and upwards. Set from C, this scale goes away from the regular major at F, who is sited almost perfectly between F and F#. Further up, the B is somewhat lower than the major, somewhat higher than the minor. This scale is used prominently in folk music all over the world, and is the mother of the blues scale.
- Modes of the Major scale, i.e. things you can play using only the white keys on a piano:
- Ionian (starts on C), the default mode of the major scale. Generally sounds happy and uplifting.
- Dorian (D), also known as Russian Minor. Minor with a sharpened 6th degree. Sounds sad but folky. Common in Irish music. Example: Eleanor Rigby by The Beatles.
- Phrygian (E). Minor scale with a flat 2nd degree. Sounds Middle Eastern, psychedelic, or Spexican due to its use in flamenco music. It also has the potential to sound really angry and/or depressed, hence its use in many styles of metal music.
- Lydian (F). Major with a sharp 4th. Sounds ethnic, jazzy or uplifting (like Ionian).
- Mixolydian (G). Major scale with a flat 7th degree. Sounds upbeat and folky. Also common in Irish music. Example: "Sweet Home Alabama" by Lynyrd Skynyrd.
- Aeolian (A), better known as Natural Minor. Sounds sad or subdued.
- Locrian (B). Minor with a flat 2nd and 5th. The tonic chord formed in this mode is diminished, so it's hard to sing or play in. Therefore, nobody uses it but Death Metal bands and jazz musicians. It is also described as sounding "evil", due to the tritone interval in the diminished tonic chord sounding dissonant, enough to give it its alternate name: the "diabolus in musica" (Latin for "devil in music").
- Related scales:
- Harmonic Minor, which is a Minor scale with a sharp 7th. Sounds Middle Eastern or suitable for Halloween/horror music. This is also what actual Russian folk and urban music uses, rather than Dorian.
- Melodic Minor, which is a Minor scale with a sharp 6th and 7th ascending and a natural 6th and 7th descending (though in actual practice, this winds up not always being the case).
- Jazz Minor, which is a Minor scale with a sharp 6th and 7th both ascending and descending.
- Lydian Dominant, which is a Lydian scale with a flat 7th. Notable for being used in the theme to The Simpsons.
- Phrygian Dominant, which is a Phrygian scale with a sharp 3rd. Sounds Middle Eastern or Jewish. Example: Hava Nagila.
- Double Harmonic, which is a Phrygian scale with a sharp 3rd and 7th. Sounds Middle Eastern or "Egyptian". Example: Misirlou by Dick Dale (and The Black Eyed Peas song that samples it, and the Lebanese folk song it was based on...)
- The whole tone scale, where all the steps between notes are whole steps. The C whole tone scale would be C D E F# G# A# C. Because of the large degree of symmetry in the scale, it sounds ambiguous and rootless. This scale is often used in visual media to indicate a transition to a fantasy or dream sequence.
- Japanese music uses a few characteristic pentatonic scales. One of these is the In scale. A D In scale is D E-flat G A B-flat. Example: The Japanese folk song "Sakura".
- Modern Classical and Jazz music use all of the above, plus any modes possible of the above, plus scales not mentioned here.