First I apologize if this is in the wrong place. I searched a couple times...because I thought I remembered a "music theory" section of this place...but I can't find it now. If there isn't one, it would be cool to have one...for me lol. I know very little about theory. I know many of the popular chords, some of them would take me a second to tell you what the 3-5-7's are etc. But I was in the studio the other day and the RE asked me what key the song was in. I grabbed the guitar off the wall, played a few things and said it was C. He wanted to be sure, so he called the owner, who is a musical genius. He's been nominated for a $%^& grammy! Anyway, he said it was Am. I don't know many keys, and I certainly wasn't doubting Brady. But for my own edification, I grabbed the guitar again, and fiddled around (I didn't know the key of Am) and said, "I'm positive Brady is correct, but it seems to work in C too?" In learning/studying the COF, I now realize, that C and Am, have the same notes in the key (I told you I was a theory idiot). So, now I see why I thought it was C. I also see now, that because I know the key of Bbm, I also know the key of G major (iirc). Neat-o. But my question is, if someone can explain it to a simpleton, how do I know the difference? When I pulled the guitar off the wall, I just played all the notes I know are in C, and listening to the song, I said, "yep that's in key, yep, yep, yep...etc." I then played some things that I know are NOT in C, and they didn't fit...so, I thought it was C. Should I have been paying more attention to, what appeared to be the root, and then compare common chord progressions? Or what? I don't know many chord progressions on paper. I'm sure that I play a lot of them, but I'm not aware that I'm doing it. I'm reading/learning all I can, but that is where I'm at right now. Never had a music lesson, just had a guitar and some drums in the basement growing up. Here is the instrumental if this helps (iirc this was it, there were quite a few beats played that night). So, what part of this is supposed to tip me off that this is not C, but rather Am?