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I see Moonlight sonata often referred to as being in the key signature of c#-minor.

I am beginning to learn music and just learning about key signatures and I would have referred to the key signature as E-major.

Is there a difference (i.e. they are en-harmonic)? Does it matter which reference is used?
How do you tell which should be used?

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The easiest way to see whether a key is Major or minor is to look at the leading tone of the relative minor and see if it is raised, In this case to we see a B# or a B natural. – Neil Meyer Jan 9 at 10:31
up vote 14 down vote accepted

Both C# minor and E major keys have the same key signature, so there is no difference there. This relationship is called 'relative key'. Each major key has a relative minor one, with the same key signature (to find it, descend a minor 3rd or ascend a major 6th from your tonic). Similarly for the minor key.

To sum up the difference: These two keys have the same key signature but different tonics.

The E major key would revolve around the note E, whereas the C# minor key would revolve around C#.

Does it matter which reference is used?

Of course it matters. A minor key sounds different than a major one, no matter what notes are included. So, you need to revolve around the minor harmony and not the major.

The relative key is not to be confused with the parallel key. The latter is when two scales have the same tonic, but one is a major one and the other is a minor one.

How do you tell which should be used?

Υοu see the harmony and the melody. In a song that is in C# minor, you are bound to see B# (leading tone) leading to C# (melody line). On the chords (harmony line) of the song you'll see V-i relationships, like G#-C#m etc. Things that will establish that the song is in C#minor.

Moreover, I'd like to add something concerning your enharmonic reference. Enharmonic note (or key signature)

is a note, interval, or key signature that is equivalent to some other note, interval, or key signature but "spelled", or named differently

A simple example: This example would be really easy to understand on a guitar or piano. C and D are a tone apart. If you ascend a semitone from C, you get C#; if you descend a semitone from D, you get Db. But both ways got you to the same note, each time spelled different. These two notes (C# and Db) are enharmonic notes.

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Thanks for your answer, just to make sure I have understood: Using just the key signature it could be either scale. By looking at the composition of the music I can deduce that in this case it is c#-minor. Getting this wrong doesn't affect playing the piece (the notes will still all be correct - sharpened by the key sig) but is relevant to understand the intent of the composition? – AntG Jan 8 at 9:58
    
Yes, you can play the song without knowing what scale it belongs in, as long as you play the notes correctly. But understanding the theory behind the piece (including but not limited to what scale the piece is in), is like understanding what a book is about and not simply reading words that are in a book. – Shevliaskovic Jan 8 at 10:36

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