Scales, & How to determine the key - Help

[thumpybumper]

Typically I find that if a song starts in, say, G, and everything resolves to G, then it's commonplace for musicians to say that that song is in 'the key of G'.

But isn't the key based on which accidentals are used?

So, for example, in the A Major scale you have A, B, C#, D, E, F#...

But if a song starts and resolves on A, and only ever uses C, D, E, F, & G - no sharps or flats - is the song in the key of A, or in the key of C (the scale which has no sharps or flats)?

Thanks in advance for helping to clear this up for me...

[Exospaciac]

Your example would be describing a song in A minor. Each major key has a relative minor.