How to know what Key a song is in on Guitar

I remember sitting up nights reading a lot of material trying to learn how to read a song, and find out which key a song is in, so that I might easily know which notes to base a solo on. But I haven't found any good guides that sums it all up, just some guides that seem to include some of the material. This is for beginners, to get a grasp of the subject. So I thought I would write a lesson just for getting the basics down. I am a beginner myself(played guitar for just over a year now, and banjo for 2 months) and love playing guitar so this has become a passion for me. Anyways, let's get down to business, shall we? Let us see, let's take a song called "Me and Bobby McGee." The version I play uses the chords G, C andD. Therefore, this song is in G. Now, why is the song in G? Well let me tell you how I knew that, because it had G, C and D in it. Let me write down a chart:

key  1   2  3  4  5  6   7     
C    C  Dm  Em F  G  Am  B-dim 
D    D  Em F#m G  A  Bm  C#-dim
E    E F#m G#m A  B  C#m D#-dim
F    F  Gm  Am Bb C  Dm  E-dim 
G    G  Am  Bm C  D  Em  F#-dim
A    A  Bm C#m D  E  F#m G#-dim
B    B C#m D#m E  F# G#m A#-dim

Originally posted on http://www.ultimate-guitar.com

In find the post interesting, so i bring it here! 

Now what can we see here? Well for startes, 1, 4and 5 are ALWAYS Major chords (G, C, F# etc). 2, 3and 6 are always minors (Dm, Am, C#m etc) and 7s are diminished chords, even though sometimes used as a major in certain songs. So, we have "Me and Bobby McGee," a song which as a G major, C major and D major chords in it. Look down the chart, which songs have G, C and D majorin it? Ah, G has those three chords. Therefore "Me and Bobby McGee" is in G. Lets take another nice country song while we're at it:

"Three Wooded Crosses" by Randy Travis The version I know of this song uses these chords: C,Dm, Em, F, G and Am (and also Am7, but yeah). So, go up the chart and tell me what key this song is in. Scroll down when you are ready. Yeah, this song is in C. Now, next song:

"Folsom Prison Blues" by Johnny Cash It uses the chords E, B and A. Which key? Right, it's in E (as most of Johnny songs there). Now the last song we'll look at is:

""Easy Come, Easy Go"" by George Strait This song used these chords: D, G, A, Em7, A7. (When facing 7's, just remove the 7). Now, this song is in D. Now you should have a clearer understanding of which songs are in which key. But, you are still wondering, why are i.e. G, C, Em and D chords in the G key, why exactly these chords? And that is where Scales come in. If you paid attention in music class in school then you will have learned the C major scale which is: C D E F G A B (C) Now this is a major scale, there are also minor scales but my knowledge of these are not sufficient to be teaching it.

 Now, you may or may not know that there are also sharps in a scale. So the full scale with sharps will be like this: C C# D D# E F F# G G# A A# B (C) This is the whole scale, notice that E and B does not have a sharp. The scale is made up by 12 notes. Which is also why you have a special mark on the twelfth fret of your guitar, if you fret the twelfth fret and play that note it is the very same note as the string unfretted. 

 The jump between say D and D# is called a halftone. If we were to jump from G note to A not, The step between that would be called a "wholetone". One wholetone = two halftones (logically) The "recipe" for a major scale isWhole:Whole:Half:Whole:Whole:Whole:Half. 

This refers to the jump between notes. Check it out: C w D w E h F w G w A w B h (C) w = whole, half = halftone. The C major scale is the only scale without Sharps(#) or flats (b). Using the recipe from above, let's construct a E scale. E w F# w G# h A w B w C# w D# h (E) (we start at E, go up a whole note, that means jumping over F and straight to F#, then a whole note jumping over G to G#. And now, we just jump a half note, over to A. Then another whole to B, and since B hasn't got any B#, we have to go directly over C as well, landing on C#. 

Then it's another jump over to D# and a halftone jump back to E.) Did you get that? Let's make a D scale using the same recipe: D w E w F# h G w A w B w C# h (D) Now, this recipe works for all the scales. There is a whole lot of information about this on Wikipedia if you just search of B major or E major etc. Now, why are these scales relevant to chords? Well, because the C scale is in the key of C. 

And if you look at the notes you play when you play say a C major chord, you will find that you play the notesC, E and G (1, 3, 5). If you play a F major chordyou'll be playing F, A and C notes. For the Am chords, you'll be playing A, C and E (1, b3, 5). If you were to play A major, you'd be playing A, C#and E. Which is why A major is not in the key of C! Starting to see a pattern now? The G chord is G, Band D notes. 

These are all notes in the C scale. Let's go back to the George Strait song, using the chords D, A, G and Em7. - D chord is D, F# and A (you're not supposed to play more than the bottom 4 strings when playing aD chord. The D string is the bass note). - G chord is G, B, D notes. - A chord is A, C# and E notes (1, 3 and 5) - Em chords is also G, B and E. A Em7 is G, B, Eand D. Sidenote: A little bit about how chords are built. Major chords are built after this recipe: 1 3 5. Meaning that an G chord are the notes G(1) - B(3)and D(5). 

Just start from G in this case and countG(1), A(2) B(3), C(4) and D(5). C = C(1), E(3) andG(5). A = A(1), C#(3) and E(5). E = E(1), G#(3) andB(5). Minor chords are assembled after this recipe: 1 b3 5. Meaning the third note is flat. Am is therefore: A(1),C(b3) and E(5) (C# flattened is C). Em is E(1),G(b3) and B(5). (Note: a seventh chord is 1, b3, 5,b7, b meaning flat 3 and flat 7).

 Now for the Strait song, we've got these notes: G,A, C, E, B, F#, D. Line em up and we've got: D, E,F#, G, A, B and C#. Now, scroll up and check which of the three scales we've cooked together adheres with this song. Yep, you're right. The D scale. 

 This eventually means, that you can solo on the song "Easy Come, Easy Go" by George Strait, if your rhythm guitarist is playing in key of D, then the notes D, E, F#, G, A, B and C would be considered "safe" notes to solo in. Even though it is safe to mix it up a little, these are ultimately the notes which you could base the solo on. Well this conludes my little lesson, hope you found it helpful and if there should be anything you need to ask about, please post a comment. Also if you have any comments about the lesson or corrections, please do not hesitate to comment, just don't flame me, I am a beginner myself. :)