Music is best explored and explained in context. So I'll use the G Major Scale as that context.

The notes ( G A B C D E F# G' ) ARE the G Major Scale. It can be anyone of the those funny Greek sounding mode names that end in ian, like Dorian, Phrygian, Lydian, Mixolydian, Aeolian, Locrian and Ionian if all notes are equal.

Even starting on one scale degree vs another doesn't define one mode from another.

• A Dorian: A B C D E F# G A'
• B Phrygian: B C D E F# G A B'
• C Lydian: C D E F# G A B C'
• D Mixolydian: D E F# G A B C D'
• E Aeolian: F# G A B C D E'
• F# Locrian: F# G A B C D E F#'
• G Ionian: G A B C D E F# G'

Most confusion when learning about modes, comes from using the same major scale as the basis for study. Like we are doing here. But for our purposes this will really help clarify why a scale is different than a mode. And, you get seven modes for the price of one major scale.

To complicate matters, there are common traditional scale names for some of the modes that are in common use.

• Dorian A B C D E F# G A' is frequently called a minor scale. As it's a minor scale.
• Phrygian B C D E F# G A B' is also a minor scale. I don't know of any other common name for Phrygian. I'm sure there are some.
• Lydian C D E F# G A B C', is a major scale.
• Mixolydian D E F# G A B C D' is often referred to as the Dominant scale and is a major scale type.
• Aeolian E F# G A B C D E' this is the same as the Natural Minor scale.
• Locrian F# G A B C D E F#' a diminished scale. No other common names that I'm aware of.
• Ionian G A B C D E F# G', this is the common Major Scale.

Dorian, Aeolian and Mixolydian are common modes in use in contemporary music. Moondance is part Dorian and part Natural Minor or Aeolian, Sweet Home Alabama is Mixolydian.

All seven modes of the G major scale are just a collection of the same notes G A B C D E F#. If all the notes of the scale are equal it is just a scale and you can name it whatever mode or scale name you like. If one note is emphasized over the other six then it is one of the scale modes.

Melodically (Single Notes)

Each mode has a Characteristic Scale Step, that when emphasized in a melody or improvised solo will give a mode its characteristic sound. You then have a scale mode or mode.

The Characteristic Scale Steps for the modes are:

• Dorian: A B C D E F# G A', characteristic scale step is F#, the sixth scale degree.
• Phrygian: B C D E F# G A B', characteristic scale step is C, the second scale degree.
• Lydian: C D E F# G A B C', characteristic scale step is F#, the fourth scale degree.
• Mixolydian: D E F# G A B C D', characteristic scale step is C, the seventh scale degree.
• Aeolian: E F# G A B C D E', characteristic scale step is G, the third scale degree.
• Locrian: F# G A B C D E F#', characteristic scale step is C, the fifth scale degree.
• Ionian: G A B C D E F# G', characteristic scale step is G, the first scale degree.

Even more to confuse you. Using triads, a three note chord, each mode has the same chords. Using our G Major Scale they are: G, Am, Bm, C, D, Em, and F#dim chords in its "chord collection".

Harmonically (Chords)

It's the characteristic scale step that identifies the primary and secondary chords for each mode. The primary chords of any one of the modes are the I chord of the mode and the major and minor triads that contain the characteristic scale step.

An example using A Dorian.

A Dorian:

• A B C D E F# G A'
• 1 2 3 4 5 6 7 8 scale degrees
• I II III IV V VI VII chord functions

The characteristic scale step of the Dorian mode is the sixth. For A Dorian this is F#. The primary chords are the I chord of the mode and the major and minor triads: Am, Bm and D. Bm is B D F# and D is D F# A. Both contains the characteristic scale step of the Dorian mode, the sixth F#. The one chord in the modes does not need to have the characteristic scale step.

• Am (A C E), the I chord
• Bm (B D F#) the II chord
• D (D F# A) the IV chord

The C, Em and G are secondary chords of the mode and the F#dim is the lone diminished chord.

OK, one more to show that the G Ionian mode is different that the G Major scale.

Characteristic scale step is the one, G. So the primary chords are the I chord of the mode and the major and minor triads: G C and Em:

• G (G B D) the I chord
• C (C E G) the IV chord
• Em (E G B) the VI chord

In the traditional key of G major the I IV and V chords are the primary chords. The tonic, sub-dominant and dominant chords.

Alright, one more example. D Mixolydian D E F# G A B C D'. The characteristic scale step in Mixolydian is the seventh, C. The primary chords are the I, V, and VII. D, Am and C.

Here is a 12 bar, D Mixolydian blues

D Mixolydian blues

||  D   |  C   |  D  |  D  |
 |  G   |  G  |  D  |  D  |
 |  Am  |  C  |  D  |  Am ||

||  I   |  VII  |  I  |  I  |
 |  IV  |  IV   |  I  |  I  |
 |  V   |  VII  |  I  |  V  ||

Each mode contains notes and chords common to a corresponding, related major scale. So it is easy to inadvertently slip into a mode's related major scale. When that happens, you have lost the mode and have actually modulated into a major key. This can occur melodically or harmonically or both. Basically avoid or handle with care the D to G as that is the V to I in our G major example.

One of the keys to all this mode or modal stuff is that the harmonic function of a chord, its I, II, III, IV, V, VI, VII is different in each mode for each chord. Here is how the Am chord functions in each mode:

• Am in A Dorian is a I chord.
• Am in B Phrygian is a VII chord.
• Am in C Lydian is a VI chord.
• Am in D Mixolydian is a V chord.
• Am in E Aeolian is a IV chord.
• Am in F# Locrian is a III chord.
• Am in G Ionian is a II chord.

Their harmonic function is completely different from mode to mode. Just like a G in the key of C is different than a G in the key of G, or G in the key of D in traditional tonic/dominant harmony.

With printed music only using key signatures for major and their relative minor keys. It's this harmonic knowledge that will allow you to determine if a song is in a major key or a mode. You might not need this to just play chords but to embellish the chords and improvise it's a definite plus.

This is a classic case for only naming things IN CONTEXT. In music there isn't a lot that can be named out of context.

Here is my secret, mystical key that I use to get all the chords from any mode based on a major scale: For triads the order of chord types are: maj, min, min, maj, maj or 7th, min, dim. For 4-part 7th chords the order is: maj7, m7, m7, maj7, 7th, m7, m7b5 or half-diminished. This allows me to remember the chords for major scales and its related modes. I can get any mode's chords from this sequence of chords by starting at a different point in the sequence. You don't have to remember so much stuff. Just this tool and your major scales.