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How to know which key a song is written in with the same key signature ? For example, the C Lydian scale has the same key signature as G major (G Ionian) AND E (natural) minor (E Aeolian) which makes it pretty difficult. Sometimes you can tell if in the bass staff, there are chords in a specific key, for example C major: CEG - CEG. But this isn't always the case. So how do you do it?

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With no key sig, the piece could be in one of several keys. C maj., and A minor being the most likely. What clues are there? In Am, there will likely be some G# notes.

However, it may also be using D Dorian, E Phrygian, F Lydian, G Mixolydian, or, unlikely, B Locrian. The melody would centre around the appropriate note, and would feel at rest on that note. For example if it was D Dorian, the last chord may well be D minor, and last note D.

That apart, there's no way of telling - unless it's announced at the top! The accompanying chords will all come from the same set, with perhaps the additional E in Am, which is where the G# came from. Otherwise, it's usually academic.

  • Thanks for your answer! But why exactly will there likely be some G# notes in Am? – Stallmp Jan 5 '18 at 11:44
  • Because in the musical world of functional harmony (as opposed to modal meandering :-) even minor keys need perfect cadences, dominant chords leading to tonic chords. And a dominant chord needs a major 3rd, the leading-note of the key. Hence the Harmonic Minor scale, which has a sharpened 7th note in addition to the key signature. It's fine to use the natural minor scale, with the flattened 7th. But if you want to work in the framework of Common Practice harmony, built on dominant-tonic relationships, you're often going to need that shapened 7th note. – Laurence Payne Jan 5 '18 at 11:51
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You decide what note the melody 'centers' on. Sometimes there's no definite answer. So you don't get to stick a definite label on the piece. It doesn't really matter, does it?

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