Keys, Scales and Modes Basic Concepts.


Brief explanation of keys and scales in music theory, what they are and how are they formed.

Key / Tonic

The principal note on which a musical composition is established is called the Key-note, or Tonic. 1

Any note can become the Tonic or first note of the Scale.


The modes are the character of the Key.2

A mode is a type of musical scale with specific features.


A scale is a series of pitches in a specific order.


Scales can be classified according to the number of different pitches they have.

Name Notes per octave Usage
Chromatic 12 Jazz; Modern Classical
Octatonic 8 Jazz; Modern Classical. (Diminished scales)
*Heptatonic 7 Most common in Western music. Diatonic scale which includes the major scale and the natural minor. Melodic and Harmonic Minor scales.
Hexatonic 6 Western folk
Pentatonic 5

Chromatic (12 notes)

It is Chromatic when the five whole tones of the Diatonic Scale are divided into semitones, so as to form twelve of these in the compass of the octave.

So a semitone in a chromatic scale is any interval between two adjacent notes. A whole tone is an interval spanning two semitones.

Relationship of intervals between the notes in a chromatic scale:

  • 0 semitones: Perfect unison (P1)
    • Diminished second interval d2
  • 1 semitones: Minor second (m2)
    • Augmented unison interval A1
      • Also known as Semitone (S), half tone (HT) or half step (HS)
  • 2 semitones: Major second (M2)
    • Diminished third interval d3
      • Also known as: Tone (T), whole tone (WT), whole step (WS)
  • 3 semitones: Minor third (m3)
    • Augmented second interval A2
  • 4 semitones: Major third (M3)
    • Diminished fourth interval d4
  • 5 semitones: Perfect fourth (P4)
    • Augmented third interval A3
  • 6 semitones:
    • Diminished fifth interval d5 or Augmented fourth interval A4
      • Also known as Tritone (TT)
  • 7 semitones: Perfect fifth (P5)
    • Diminished sixth interval d6
  • 8 semitones: Minor sixth (m6)
    • Augmented fifth inteval A5
  • 9 semitones: Major sixth (M6)
    • Diminished seventh interval d7
  • 10 semitones: Minor seventh (m7)
    • Augmented sixth interval A6
  • 11 semitones: Major seventh (M7)
    • Diminished octave interval d8
  • 12 semitones: Perfect octave (P8)
    • Augmented seventh interval A7
Diatonic (7 notes)

The Scale is Diatonic when the five whole tones and two semitones which compose it, succeed each other regularly and in the natural order, whether ascending or descending.

There are two popular diatonic scales: Major and minor scales

In the Major scale the sequence of intervals between the notes is:

whole, whole, half, whole, whole, whole, half.

The scale degrees, i.e.: the position of a particular note on a scale relative to the tonic, are:

  • 1st: Tonic (I)
  • 2nd: Supertonic (ii)
  • 3rd: Mediant (iii)
  • 4th: Subdominant (IV)
  • 5th: Dominant (V)
  • 6th: Submediant (vi)
  • 7th: Leading tone (vii° or bVII)
  • 8th: Tonic
Degree Roman Numeral based on (major / minor) scale Name Role/Name Explanation
1st I / i Tonic
  • The key note
  • The tonic is the note upon which all other notes of a piece are hierarchically referenced
  • The tonal center or final resolution tone that is commonly used in the final cadence in tonal music. def
2nd ii / iio Supertonic super- + tonic: one step on top of the tonic
3rd iii / III Mediant
  • From Latin: to be in the middle
  • the middle note between the tonic and the dominant med-def
4rd IV / iv Subdominant sub- + dominant: the note one step below the dominant sub-def
5rd V / v Dominant "called the dominant because it is next in importance to the first scale degree, the tonic"
Degree Roman Numeral Name Role/Name Explanation
6rd vi / VI Submediant The lower mediant
7rd viio / VII Leading Tone "a note or pitch which resolves or "leads" to a note one semitone higher or lower, being a lower and upper leading-tone, respectively"lead-tone
How to read the above table? For example, the *submediant* is the *sixth degree* of the *diatonic scale*.

Tonic and root should not be confused:3

The tonic is the reference note of a scale, while the root is the reference note of a chord.

The roman numeral analysis is shown in parentheses.4. Lower-case Roman numerals indicate minor triads and upper-case numerals for major triads, and degree (o) and plus (+)signs.

The triads, i.e.:the set of three notes that can be stacked in the Staff vertically in thirds, built on each scale degree follow a distinct pattern.

  • 1st: Major triad (I)
    • Major chord: a chord that has a root, a major third, and perfect fifth
  • 2nd: minor triad (ii)
    • Minor chord: a chord that has a root, a minor third, and a perfect fifth
  • 3rd: minor triad (iii)
  • 4th: Major triad (IV)
  • 5th: Major triad (V)
  • 6th: minor triad (vi)
  • 7th: diminished triad (viio)

In a Major Triad, its third is major or composed of two full tones.

While in a Minor Triad, its third is minor, or composed of a tone and a semitone.

In the Minor scale, the sequence of intervals between the notes is:

whole, half, whole, whole, whole, half, whole

In the Diatonic Scale, the two semitones are found between the third and fourth, and the seventh and eighth degrees, in the major mode, and between the second and third, and the seventh and eighth, in the minor.

The Scale of C Major, may serve as a model for all the other major scales.

Scale of A Minor will serve as a model for all the other minor scales.


A chord is a “harmonic unit with at least three different tones sounding simultaneously”.5

A chord can be constructed starting from each note of a scale.

So a chord is built upon some note (e.g.: the C chord is built upon the C note), and can be of some type:

  • major
  • minor
  • diminished

Or in a progression we can have more than three notes:

  • augmented,
  • 7th

Each chord of the scale can be described with the number of the corresponding note scale degree, using Roman numerals.

Diatonic Harmonization

If we have a diatonic scale (7 notes) we can have seven basic diatonic triads(chord formed with three notes), using each note of the scale as the root of the chord.

The diatonic harmonization of a major scale will have three major chords and three minor chords.

In a major key:

Roman numeral in a major key Chord Name Chord Type
I Tonic chord Major chord
ii Supertonic chord Minor chord
iii Mediant chord Minor chord
IV Subdominant chord Major chord
V Dominant chord Major chord
Roman numeral Chord Name Chord Type
vi Submediant chord Minor chord
vii° Dominant seventh chord Diminished chord (two minor thirds above the root (minor triad with a lowered (flattened) fifth))

In the relative minor key, major and minor chords, will keep their type but each degree of the relative minor key will be now according to the relative minor scale, i.e.: the first degree of A minor scale (relative minor of C major) is A minor chord.

For example, the relative minor of C major is A minor (the 6th degree), and in the key of A minor:

  • the i chord (first degree) is A minor
  • the iv chord is D minor
  • the v chord is E minor

In a minor key, the third of the dominant chord (v) is often raised by one semitone to form a major chord (or a dominant seventh chord if the seventh is added).

The vii seventh degree of the scale, also known as the leading tone because it resolves or leads to a note one semitone higher, to the tonic, forms a diminished chord (viio).6

In a minor key:

Roman numeral in a minor key Chord Name Chord Type
i Tonic chord Minor chord
ii° Supertonic chord diminished chord (two minor thirds above the root (minor triad with a lowered (flattened) fifth))
III Mediant chord Raised by one semitone to form a Major chord
iv Subdominant chord Minor chord
v Dominant chord Minor chord
Roman numeral Chord Name Chord Type
VI Submediant chord Major chord
VII Dominant seventh chord Diminished chord

Also kwon as Chord Progression.

Songs with two chords:

  • I-V progression
    • Rancheras
    • Jota

Songs with three chord types:

  • I-IV-V chord progression
    • simple traditional music
    • folk music
    • rock and roll (popular / basic)
    • blues (popular)

Songs with four chords:

  • 1-5-6-4 progression
    • pop (commercial)

Key signature

In music notation, the Key of a piece of music is indicated by the number of Sharps or flats which are found at the Clef.

Relative keys

Each major Key, also called the principal Key, has its Relative Minor Key.

Keys which share the same key signature are called Relative Keys.

Modern Modes

Modes were inspired by ancient Greek music theory; the word mode is used mostly to refer to any of the Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, or Locrian scales.

Modern Western modes use the same set of notes as the major scale, in the same order, but starting from one of its seven degrees in turn as a tonic, and so present a different sequence of whole and half steps.7

Each mode has an associated character.

Mode Tonic relative to major scale Interval sequence Example Character
Ionian I W–W–H–W–W–W–H C–D–E–F–G–A–B–C Serious/happy
Dorian II W–H–W–W–W–H–W D–E–F–G–A–B–C–D Sad
Phrygian III H–W–W–W–H–W–W E–F–G–A–B–C–D–E Mystic
Lydian IV W–W–W–H–W–W–H F–G–A–B–C–D–E–F Tender
Mixolydian V W–W–H–W–W–H–W G–A–B–C–D–E–F–G Happy
Aeolian VI W–H–W–W–H–W–W A–B–C–D–E–F–G–A Tearful
Locrian VII H–W–W–H–W–W–W B–C–D–E–F–G–A–B Perfect

Modes, chords and scales

Every chord has an implicit scale depending of its role in a song.

  • If we are in C major key, C major chord (the first degree) has an implicit scale we can play on, the Ionian scale
  • In C major key, D minor chord implicitly has the Dorian scale
  • E minor, has associated the Phrygian scale
  • F major, has associated the Lidyan scale
  • G major, Mixolydian scale
  • A minor, Aeolian scale
  • B dim, Locrian scale


There is a norm we can use to decorate a song we are composing based in the above modes principles.

We notice that Some of the above scales has 2nd minor and some 2nd major.

A norm says that we can, over any triad chord, do a Legato8 between second and third degree, if the 2nd degree of the implicit scale in that chord, is a 2nd major, so we can do it always expect on the 3rd and 6th degree.9


Tonic def.
Med. def.
  • Benward & Saker (2003), p.32.
  • Benward & Saker (2003). Music: In Theory and Practice, Vol. I, p.33. Seventh Edition. ISBN 978-0-07-294262-0. “So called because its function is next in importance to the tonic.”

  1. “Of the Tone and Mode or Key” New and Improved Method for the Guitar, by Matteo Carcassi, Boston : Oliver Ditson & Co, 1853, p. 7.,  ↩︎

  2. “Of the Tone and Mode or Key” New and Improved Method for the Guitar, by Matteo Carcassi, Boston : Oliver Ditson & Co, 1853, p. 7.,  ↩︎

  3. Wikipedia contributors. (2020, May 29). Tonic (music). In Wikipedia, The Free Encyclopedia. Retrieved 20:33, June 29, 2020, from ↩︎

  4. ↩︎

  5. Benward & Saker (2003). Music: In Theory and Practice, Vol. I, pp. 67, 359. Seventh Edition. ISBN 978-0-07-294262-0. ↩︎

  6. Wikipedia contributors. (2020, June 21). Chord progression. In Wikipedia, The Free Encyclopedia. Retrieved 19:39, June 29, 2020, from ↩︎

  7. ↩︎

  8. ↩︎

  9. ↩︎

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Basic keys, scales and modes concept in music. What they are and how they related to each other

Basic musical concepts. Understand musical terminology and how concepts relate to each other.
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