Is there a rule to go by when a guitar player is using a capo and they are playing their chords in the key of A, D G or C shapes to know what key they are playing in.
Finding the key
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Yes. Every fret on the guitar is a half step in music. A half step is just the next letter in the sequence below:
G, Ab, A, Bb, B, C, Db, D, Eb, E, F, F#
Count the number of frets up to where the guitarist placed their capo, and you’re counting the number of half steps to find your key. So if the guitarist is playing in the G position, and puts their capo on the 2nd fret, that’s 2 half steps higher than open position and they’re going to be in A. If they’re playing in D position and have their capo 3 frets up, that’s 3 half steps higher and they’ll be in the key of F. If they’re playing in C position and have their capo on the 1st fret, that’s going to be Db. And so on…
Guitar capo language is actually not as hard as it seems, once you get used to it and start recognizing the often-used keys. The 4 chord shapes you mentioned are going to be the main chord positions played out of (more specifically, just G and C), so all you have to do is get used to counting the frets and the corresponding musical half steps.
Make sure you memorize that sequence above because not every letter has a flat between them; for example, one half step up from B goes straight to C. When I first started using a capo I kept a paper chart in my guitar case.
Hope that helps!
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Nice post Libby. I’ll add one little thing regarding memorizing the notes. For some folks, seeing the notes on a piano keyboard has proven very helpful. No black key between two white keys (like E to F and B to C) means there is only a half step between those notes.
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@Mike_R great point. I learned whole & half step theory on a piano keyboard originally. 
By the way to clarify from my first post, a WHOLE step is climbing 2 numbers in the pattern or 2 frets on the guitar.
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Hello Annie,
Seen some great responses so far for you. You could back all this up by putting some stickers on the top of the neck for a while that gives you top and bottom string (Standard Tuning)and adds visual reinforcement.
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Flatpicken_Libby your explanation was great. It made it so much easier for me to understand. I actually started to quiz myself by choosing a fret and chord shape and checked to see if I figured it out correctly using the capo chord chart from Chuck_R. Thank you all for your help.
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