Hello everyone yesterday i was reading a book about music theory

and so far i have understood that the G key is the most used in guitar, isnt it?

So, i have some questions:

1)is there only one note C in the G key? and in what fret do i play that note?

2)is there any rule to know in what fret i need to play the notes in the staff? or the only way is to memorize every note in the fret?

sorry for that questions, they're maybe stupid
thank you
Well, yes. G major is very used, but mainly E minor which is its relative Minor. For your next questions:

1. There isn't one note in the key of C, there are 7 like every other key. There just aren't any flats or sharps, meaning the notes of C major / A minor would be C D E F G A B.

2. I'd learn both. Once you memorize every note on the fretboard, you could apply that knowledge to sheet music. Meaning, if you see a high F on sheet music, you'd know several positions to play that note in.
I agree with Kartman, but I think the answer you were looking for in your first question is that there are an infinite amount of C notes in the key of G. The key of G contains the notes G A B C D E F# and then starts again on G one octave higher than the first one. This means that if you play through the key again, you will hit another C note that is one octave higher than the first one and if you keep on doing this, you will soon realize that it will never end.
thats what i wanted to know, and for example:

how do you know where to play that exactly C note?
play it anywhere...a C is a C is a C.

Scales exist all over the fretboard...wherever there's a G there's a G major scale, and the corresponding C.
Actually called Mark!

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