#1
i am an absolute noob when it comes to theory and applying it onto the guitar. I am wandering how can I work out what key I am in when I play a song.
#3
what he said plus: its the chord that the song sounds like it should end on. also if you look at a theory article it will list chords that can be in each key, and that should help. if i were you i'd learn a bit of guitar-related theory it helps with things like this.
#5
two ways.

Write down the notes you are playing and match it up to a key. You will usually get there by paying attention to how many sharps or flats you're using
no sharps = key of C major/Aminor
1 sharp = key of G major/Eminor
etc round the circle of fifths.

Or look at the types of chords. The I IV and V chords in a major scale are always major triads in diatonic harmony. The ii iii and iv chords are always minor. If you look at the chords you are using and can match them up to that pattern you should be able to identify the root of the parent major scale.

Of course you will want to pay attention to what chord in the progression provides resolution.
Si
#6
Quote by 20Tigers
two ways.

Write down the notes you are playing and match it up to a key. You will usually get there by paying attention to how many sharps or flats you're using
no sharps = key of C major/Aminor
1 sharp = key of G major/Eminor
etc round the circle of fifths.

Or look at the types of chords. The I IV and V chords in a major scale are always major triads in diatonic harmony. The ii iii and iv chords are always minor. If you look at the chords you are using and can match them up to that pattern you should be able to identify the root of the parent major scale.

Of course you will want to pay attention to what chord in the progression provides resolution.
That will determine if it's say G major or E minor. They both have one sharp (F#) but they are different. Knowing which chord provides resolution tells you which one it is. If the chord would be Emin or E5 or some crap then the key would be E minor.


+1 to everything else you said (i just had to elaborate on that)


Edit: Oh and if you go by that dumbass "general rule" some nimrod came up with that says it's usually the first chord, I WILL come to where you live and strangle you in your sleep. That's not how you identify a key.
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#7
The "first chord rule" works a majority of the time. The only time it won't work, is if a song is in the key of C and it starts on Dm, which is the ii of C. So, it works like 95% of the time, but there's going to be a few songs that will trick you. I've seen it happen.

Another good method of finding the key, is to play notes along a single string. It doesn't really matter which one. I used to use the 5th or 6th string. Play the notes, one at a time, along that string until you find the note that seems to drone, or "fit in" with the song. There may be a couple of them that seem to "fit", but one should really stand out. This note is likely going to be your key. Now, you have to decide if the song is major or minor. As you get better over time, you'll be able to recognize major and minor just by listening to the song, but for now, you'll have to determine it by playing a scale and hearing if it sounds good, or not. If you're unfamiliar with Pentatonic Major, Pentatonic Minor and the Major scales, just do a Google search and you'll find plenty of info. Unless a song changes keys or you're playing along with some Country tune that bounces around all over the place in Pentatonic Major, this advice will work well for you. I really don't want to confuse you, but at some point you're going to hear someone mention something about Relative or Natural Minor. Am is the Relative Minor scale to C Major. A Relative Minor scale is built from the vi degree of the Major Pentatonic scale. The Relative Minor scale uses the same notes as parent scale it's taken from - it just starts out at a different note. If that last part sounds like gibberish to you, don't worry about it. You'll understand it soon enough.
#9
The only time it won't work, is if a song is in the key of C and it starts on Dm, which is the ii of C.


That's an oddly specific exception.

The chord the song starts on has, inherently, nothing to do with the key whatsoever. Whether or not to start a song with the tonic chord is entirely up the composer, and starting with other chords is very common.
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