For guitarist, from beginners to advanced players, there is a chord system called CAGED system, where you take the form of C, A, G, E, D barre chords and move them across the fretboard over different roots.
(img from https://www.cagedguitarsystem.net/)
So you only need to learn at most 5 basic chord shapes to start playing chord progressions on the guitar. However, for pianists, this concept might be a bit difficult to understand without basic guitar knowledge, and learning the guitar for a short time isn't always the most efficient and effective method of approach to arranging for chordal playing on guitar. Fortunately, I have recently analyzed this system a bit more, and found an interesting pattern within these chords.
If we take the voicing of these chords along, without considering their fingerings on the guitar, there are really only two voicing patterns. Let us decode these two in scale degrees, notation, and piano roll, building upon the root C.
Shape 1 (EAD): 1 5 1 3 5 1
Shape 2 (GC): 1 3 5 1 3 5
These numbers are labeled as scale degrees from the major scale. Any variation would be an alternation, explained later in this article.
Shape 1 (EAD):
Shape 2 (GC):
Shape 1 (EAD):
Shape 2 (GC):
The shape names within the bracket are arranged by the number of notes they contain. In the first shape, E chord would contain all 6 notes, A chord would contain 5 notes starting from the root at the beginning, while the D chord only contains the first 4 notes. Same logic applies to the GC shape.
With the above knowledge as the foundation, we can begin to experiment altering the shapes for more expressive chord voicings.
Because of the physical limitations of the guitar (only 6 strings with fixed interval relationships) and guitarists (we only have 4 fingers on one hand to press the string, 5 max), alternations are not always an easy reach. I have found it is best to keep the alternations for each shape within 3 notes. Once you get familiar with the basic guitar voicings, you can try combining multiple alternations as one, such as altering two or three sequential strings by the same amount of semitones (in some situations it only takes one finger to do that).
Every note within the shapes (except for the left most root I guess) can be altered, some by a semitone, some a whole tone, some even a minor or major third. Below is a list of changes each note can be altered to, colored by their difficulty: green is common and simple, yellow means less common, and red indicates that a certain amount of finger stretching is required, which might not co-exist with alternations on other strings.
Note: I did not write them as actual playing position since we are covering multiple guitar shapes with one voicing.
Note 2: It's quite difficult to play minor chords with Shape 2, we normally use Shape 1 or other voicings for minor chords. The only way to play a minorish chords is to make Shape 2 a minor 9th or minor 11th, alternating +4 semitones for the 3rd and 4th string, which is a common voicing.
Using this method of alternation, we can already build few extended chords on guitar. For example:
A7 (Shape 1 A, 1 alternation)
C9 (Shape 2 C, 2 alternations)
Gmaj13 (based on Shape 1 E, 3 alternations)
Fmaj9 (based on Shape 1 D, 2 alternations)
With all the chord voicings available built from these two shapes alone, we can already construct guitar chordal playing arrangements that is not only convincing sound wise but also playable. The trick here is to not play everything. Omit some notes, leave some spaces. Not all chords require all six strings. It may be difficult to skip strings while strumming, but you can always limit the range of strummed notes. If you are finger picking, string choice becomes quite easy to handle.
Here's the score. I indicated which shape each chord belongs to, marked as "S#", plus number of alternations from the basic shape, marked as "a#".
Click for a bigger svg image.
Now it's your turn, come up with something musical with these two shapes.