"The Key of ?"

The key signature actually refers to how many and which flats and sharps go in a song. It helps, because for a given key signature, there are certain chords that sound good (and some you can sneak in).

For example (basic example): In the key of G there is 1 sharp (F sharp). The most used chords in the key are G, Em, C, and D. There are others that go with it, but all the chords are made up of the notes G,A,B,C,D,E, and Fsharp.

If someone just says a song is in the key of G, you can assume that it uses the G major scale. Of coarse, if they say G minor, it would be the G minor scale and a little different set of notes.

That's very very basic though. I'm sure someone here can explain it much better than I.

To over simplify it look at what chords are used.
See how they fit together.
Take the chords apart (do any of the chords have sharp or flat notes?) what key signature fits with them. That is very basic. Others will have you look at standard chord progressions and fit the chords within them.
Quite often the first chord in a song is the key of the song so a good place to start looking but since it is not always true do not use that as an automatic basis for naming the key.
lets take an example;
Chords are G, Am, and D The G and Am both have all natural notes but the D has an F# so it is likely the key of G
Same progression but this time the A is major G,A,D we already know the D has an F# in it. the G has all natural notes but now the A has a C# so with both F# and C# i would say we are now in the Key of D (even though the first chord was G)
Note: sometimes chords outside of key will be thrown in, be careful of this.
Edit: when I said progression I was reffering to the way the chords showed up in the piece. an acyual progression would be different. In example 1 the progression would be I,ii,V (a bad example) the second would be I,IV,V (D,G,A a better example)

I think this topic has really been beaten to death on this board. Just use the search function and you can find 20+ threads on key signatures.

I think this topic has really been beaten to death on this board. Just use the search function and you can find 20+ threads on key signatures.
Yea but we are all musicians and love to talk shop. Can't get us to shut up for second.

Yea but we are all musicians and love to talk shop. Can't get us to shut up for second.

LOL. So true. :D :D :D
how can you use it when you're playing?

When you know which key you're in you can throw in extra notes and other such dodges or embellishments. On occasions you can even swap chords for others that still fit.

The key also gives you a set of notes to use if you're improvising a solo - the scale for that key.

Thanks for lots of informative answers :)

Just use the search function and you can find 20+ threads on key signatures.

yeah and right here is one of those places

can you change the key of a song if you raise or lower all the chords one?
so if it goes

G C D then

A D E

Yes, however I have noticed that sometimes it doesnt sound quite right. All the notes are there, but something isnt quite right about it to my ear. I could just be insane though.