[atari 2600]

A chord is the more or less simultaneous sounding of three or more notes.

For instance, the C major chord triad is a C, E, and a G note (the root (in this case, C), the third (E) and the fifth (G) on the diatonic scale starting with C (I-III-V)).

And then you would just have to be acquainted with the chromatic scale and based on how the guitar is tuned at the nut find the notes necessary to make the chord or at least a triad (bearing in mind that any open strings played count as the note of the open string at the nut).

Check out a circle of fifths for a further understanding of what scott v is getting at with the keys. Whie it is certainly not altogether necessary to know the specifics of theory, such an understanding can help make a good song that much more exciting. In general this seems especially true when one is puzzling over what the third progression of a tune or middle eighth part is going to be (since it is usually intuitively simple to write a couple of progressions/parts, and a lot of times it is the dominant chord to the tonic chord, but coming up with that third progression and/or riff, or any more than that, can be perplexing).

Hope this helps to point you in a direction to search the internet. Most alternate tunings are comprised the way they are to make the fingering of the chords easier anyway.