Chords are written out in Chord Boxes which are a visual representation of the guitar fretboard.
They look like this:
Now check out the two images below to make a bit more sense of what going on:
Chord Box Diagram
The Guitar Fretboard
Chord Box Key
1. The lines going downwards represent the strings. These lines are all of a different thickness, just as the strings are on your guitar.
2. The double line going across the top of the image represents the Nut.
3. The 3 single lines going across represent the fret wires.
4. The spaces in between the nut and each fret wire represent the fret number.
Finger Numbers
Within a Chord Box, you’ll also see blue circles with numbers inside. These represent the fingers you press each note down with. Like so:
Symbols
And finally, you’ll also find specific symbols placed on the open strings. These symbols tell you what to do with each open string that you’re not fretting (pressing down).
Symbols Key
1. The “0” means you need to strum this string.
2. The “X” means that you must either mute or NOT strum this string.
3. The “?” means that you can strum this string if you choose to. It’s optional.
Also, any notes that you fret (press down) with the left-hand fingers, must always be strummed.