Sorry if what I'm asking is super stupid, but I simply don't get this.

If you play a regular C chord on the guitar (where you have your index finger on 2nd string 1st fret, middle finger on 4th string 2nd fret and ring finger on 5th string 3rd fret) and then put on a capo and play a C chord the same way, are you then playing in another key?

When/how do you know what key you're playing in?

I don't get keys at all and every time I try reading about them I think I get it until I think about it and realize that I don't get it AT ALL!! ARG!!!
Yeah shifting it up would technically change the chord you're playing, even if it's the same shape. For example putting a capo on the 2nd fret and playing on open C would make it a D chord.

To answer you're other question, you need to find out what notes your playing to work out what key you're in. It takes practice. Search the threads and possibly even compile a cheat sheet so you can work out the key when you need it, and then eventually you won't need it.

So yeah, GTMF and search the UG forums.
you're not playing in another key exactly, you're just playing a different chord. Which chord that is depends on where you put the capo...do you know anything about chord construction yet?
Actually called Mark!

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Let's put it this way:

If you have 2 guitars, 1 with a capo on the second fret and one without capo. If you play a 'C' on the guitar with the capo, this will sound the same as a D played on the guitar without the capo. The shape is C, but you're actually playing a C + 2 frets = D.

When you put your capo on 4 and you put your fingers in the shape of a C, you're playing a C + 4 frets = E

And so on.
Thank you so much for your replies, I think I'm actually starting to get it now...

@Heavens to Hell: Stupid question - I know - but how should I go about making a cheat sheet for this?

@Steven Seagull: Nope, I don't know anything about constructing chords (yet...)
its all baised off one simple scale its 1 2 3 4 5 6 7 8 (major scale)but then theres flats and sharps. every number flat and sharp is a fret

1 | b2 | 2 | b3 | 3| 4 | b5 | 5 | b6 | 6 | b7 | 7 | 8 thats 12 notes

just write it down ull get it..thats what i did

1 3 5 major chord
1 b3 5 minor chord
1 b3 b5 diminished chord

then theres 7ths but i think thats enough for now maybe someone else can explain it better but thats really very simple

maybe i should have just made a lesson since i wrote so much lol