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How does one go about finding the relative keys of modes?

For example, determining the relative major of F# Phrygian or determining the relative locrian of G Mixolydian?

  • I would probably first find the tonic in all cases.Seems convenient to me. – alexsms Dec 19 '18 at 7:46
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There are a couple of strategies you could use.

  1. Work out the key signature of the starting mode in question & then ascertain the required mode. In your 2 examples, F♯ Phrygian has a key signature of F♯ & C♯, so its relative major is D Major; G Mixolydian has an empty key signature, so the Locrian mode with the same key signature is B Locrian.
  2. Alternatively, use the your knowledge of which scale degrees in the Major scale for the tonics of each mode & work it out from there. You know that the mode mappings on major scale degrees are:

    I   (tonic)        = Major/Ionian
    II  (super-tonic)  = Dorian
    III (mediant)      = Phrygian
    IV  (sub-dominant) = Lydian
    V   (dominant)     = Mixolydian
    VI  (sub-mediant)  = Minor/Aeolian
    VII (leading-note) = Locrian
    

So in your examples, the Phrygian mode is formed on the mediant (3rd degree) of the major scale, so the "relative major" of F♯ Phrygian is 2 scale steps back in the Phrygian mode (F♯→E→D); the Mixolydian mode is formed on the dominant (5th degree) & the Locrian on the leading-note (7th degree), so the "relative Locrian" of G Mixolydian is 2 scale steps up in the Mixolydian mode (G→A→B).

EDIT: additional answer

A third alternative is to learn the relations between the modes. Just like the relation between the major & relative minor is a 6th, all of the other modes can be related in a similar way. Here's how it would look:

Table interval relations between modes & their relatives
  Read across to the starting mode & down to the relative mode. 
  The resulting interval is the distance between the tonics of the relative modes.

> Starting Mode:     Major | Dorian | Phrygian | Lydian | Mixolyd | Aeolian | Locrian
Relative Major       same  |↓Maj 2nd| ↓Maj 3rd |↓Prf 4th|↓Prf 5th |↓Maj 6th |↓Maj 7th
Relative Dorian    ↑Maj 2nd|  same  | ↓Maj 2nd |↓min 3rd|↓Prf 4th |↓Prf 5th |↓Maj 6th 
Relative Phrygian  ↑Maj 3rd|↑Maj 2nd|   same   |↓min 2nd|↓min 3rd |↓Prf 4th |↓Prf 5th
Relative Lydian    ↑Prf 4th|↑min 3rd| ↑min 2nd |  same  |↓Maj 2nd |↓Maj 3rd |↓Aug 4th
Relative Mixolyd   ↑Prf 5th|↑Prf 4th| ↑min 3rd |↑Maj 2nd|  same   |↓Maj 2nd |↓Maj 3rd
Relative Aeolian   ↑Maj 6th|↑Prf 5th| ↑Prf 4th |↑Maj 3rd|↑Maj 2nd |  same   |↓Maj 2nd 
Relative Locrian   ↑Maj 7th|↑Maj 6th| ↑Prf 5th |↑Aug 4th|↑Maj 3rd |↑Maj 2nd |  same 

So, in your examples we read across to Phrygian in the top line & then down to the Relative Major row & find the interval between the tonics (key-notes) is a Major 3rd, going down for Phrygian to Major. Similarly, Mixolydian to Relative Locrian is up a Major 3rd.

  • Quite helpful. But just wondering does this task often arise in real practice? As a guitar player I find that probably it's practical to view G-Myxolyd. as degree V (dominant) of Cmaj, hence work from Cmaj as basis for all other modes. Or view Fm# as degree III of Dmaj and think of Dmaj as tonal centre (without thinking of key signature, because it's probably not easy to quickly recall that F# Phryg. has two ##s unless one improvises in different Phryg. keys). – alexsms Dec 19 '18 at 7:38
  • @alexsms as a guitar player, I have the same issue. It wouldn't be that easy to recall the number of sharps in different modes – noorav Dec 19 '18 at 7:51
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    @noorav I also play the piano, so I remember (or can quickly tell, simple methods) all key signatures, but it's usually just the tonic. – alexsms Dec 19 '18 at 7:56
  • @alexsms, I've written up a lookup table for you. I can't think of a way to do this without memorising the relationships. My background being art music, i learned all of these relationships via scales, keys, key signatures & intervals. I can offer no insight into tricks for musicians who don't work with those building blocks as instant recall. – Dean Ransevycz Dec 19 '18 at 8:46
  • @DeanRansevycz, thanks. I guess one could use it too to better remember (understand) some chord substitutions. – alexsms Dec 19 '18 at 8:59
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Given that a 'relative key' is another that contains the same notes, as in A minor is 'relative' to C major.

The simplest has to be go to the parent key of the mode. Taking your example of F# Phrygian we go back to parent D major. So 'relative' modes for this example are E Dorian, G Lydian, A Mixolydian, etc. Thus making the relative Locrian to G Mixolydian B Locrian.

Your G Mixolydian example takes us to C major, which gives us D Dorian, E Phrygian, F Lydian, etc.

Knowing key sigs will help with #/b, and it's important to know those anyway.

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