Key Signatures - Major and minor

[ambergahill]

I am learning about key signatures. First we learned about the 15 major keys. I fully understand this. Then they went on saying that there are 15 minor keys, all of which have the same signature as a major key. My question is why have these minor keys at all? How do you even tell by looking at something whether its a E minor key or G major key? I am confused on the point of minor key signatures. Please shed some light on this! 

[gaboriau]

Hi !  

First sorry for my very bad english !

I'm a beginner too (1 year now).

why have these minor keys at all?

Because it's not the same "color" (tonality). The tonic it's not the same.

(...) There can be major scales and minor scales. The tonic remains the same in these two different "modes," for a given key, whereas scale degrees such as the third degree and the sixth degree are altered in the minor scale.

This can be seen another way. Each minor scale uses exactly the same set of notes (key signature) as some major scale and vice-versa. The only difference is which of these notes functions as the tonal center — which of them is the tonic. For example, C major and A minor have no sharps or flats. Consequently, the tonic plays an important part in determining why music composed using a minor mode sounds different from music composed using a major mode.
https://en.wikipedia.org/wiki/Tonic_%28music%29

https://en.wikipedia.org/wiki/Tonality

Try to improvise something in A minor, and then in C majeur. Play in A minor for exemple mean you are often play the tonic A, and the central cadence I-V it's not the same.
https://en.wikipedia.org/wiki/Chord_progression

Also you have Natural, Melodic and Harmonic minor.
https://en.wikipedia.org/wiki/Minor_scale

For resume, the key signature help, but it's not the Scale. You can have all Greek Scales in the key of C major.
https://en.wikipedia.org/wiki/Musical_mode

You can have many other scale you can imagine in C major key signature (Pentatonic C-D-E-G-A for example). In others words, you can play in a C major tonality with any key signature if you put "naturals" on flat and sharp. 

[nanabush]

The tonal center usually helps decide a key.  For example, if it starts and ends on E, has one sharp in the key signature, and you see some D#'s and occasionally C#'s, it's probably in E minor.  Also, in a G major chord, there isn't an E, so it's not even beginning on a note found in the chord of G major. 

That's a very basic example, but is a good starting point.

You should also hear the major and minor quality to a piece if it is played for you.

[pianoladywong]

Every major key has a relative minor. The 6th degree of a major key is the relative minor. For example, if you are in the key of C. The 6th degree is A, hence A minor sharing the same notes as C major is the relative minor. The thing to remember about minor scale is the steps between each note - whole, half, whole, whole, half, whole, whole. E minor will be E, F#, G, A, B, C, D, E. I would recommend remembering the relative minor of each major scale.

Most music starts with the tonic or the root. If the key signature has a F#, most likely the piece is in G major or E minor. If the 1st chord is E minor, or the melody starts with E, then most likely you are in E minor. You should also look out for accidentals. If you see a D#, which is the leading tone and the next note is E. This may very well implies a harmonic minor scale or B7 chord, which is a V-I cadence. This will let you know that we are in the key of E minor instead of G major.