In any key it is possible to build chords on each note of a scale. This means that for every major scale there are seven possible chords which can be used for creating guitar parts and harmonising melodies. These seven chords are called scale tone chords. It is common practice to describe all the chords within a key with Roman numerals. The example below demonstrates the seven scale tone triads (three note chords) in the key of C major.
Once you know the notes for any of these chords, you can use any fingering you know to play them. This may involve re-arranging the order of the notes or doubling some of them, but as long as the chord shape contains only the three note names of that particular chord, you are still playing the right chord. The next example demonstrates a common progression in the key of C This progression would be described as I III IV V III VI II V.
The rhythm notation indicates that you are strumming chords, but you could also play this (or any) progression as arpeggios - experiment!